Dioxins and dioxin-like compounds in thermochemical

conversion of biomass

Formation, distribution and fingerprints

Qiuju Gao

Doctoral Thesis, Department of Chemistry

Umeå University, 2016

This work is protected by the Swedish Copyright Legislation (Act 1960:729)

ISBN: 978-91-7601-451-6

Cover picture: WordleTM

Electronic version available at http://umu.diva-portal.org/

Printed at the KBC Service Centre, Umeå University

Umeå, Sweden, 2016

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Table of Contents

Abstract 1 Sammanfattning (Summary in Swedish) 2 List of Abbreviations 4 List of Publications 5 1. Introduction 7 2. Background 10

2.1. Biomass for energy production 10 2.2. Thermal decomposition of biomass 11 2.3. Thermochemical conversion 13 2.3.1. Pyrolysis 14 2.3.2. Torrefaction 15 2.4. Dioxins and dioxin-like compounds 16 2.4.1. General description 16 2.4.2. Toxicity of dioxins 17 2.4.3. Formation mechanisms 19

3. Materials and methods 24 3.1. Feedstocks 24 3.2. Experimental setup of microwave-assisted pyrolysis and torrefaction 27 3.3. Sample extraction and cleanup 30 3.4. Instrumental analysis 30

4. Results and discussion 32 4.1. Solvent effects in pressurized liquid extraction 32 4.2. PCDDs, PCDFs and PCNs in microwave pyrolysis 37 4.2.1. Levels and relative distributions 38 4.2.2. Homologue profiles 40 4.2.3. Isomer patterns and formation pathways 42 4.3. Dioxins in torrefaction products 49 4.4. Toxicity equivalent values 54 4.5. Effects of chemical composition of feedstocks 55 4.6. MAP vs torrefaction: the importance of physical dynamics 56

5. Conclusions and future perspectives 61 Acknowledgements 64 References 65

1

Abstract

In the transition to a sustainable energy supply there is an increasing need to use biomass for replacement of fossil fuel. A key challenge is to utilize biomass conversion technologies in an environmentally sound manner. Important aspects are to minimize potential formation of persistent organic pollutants (POPs) such as dioxins and dioxin-like compounds.

This thesis involves studies of formation characteristics of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and naphthalenes (PCNs) in microwave-assisted pyrolysis (MAP) and torrefaction using biomass as feedstock. The research focuses are on their levels, distributions, fingerprints (homologue profiles and isomer patterns) and the underlying formation pathways. The study also included efforts to optimize methods for extracting chlorinated aromatic compounds from thermally treated biomass. The overall objective was to contribute better understanding on the formation of dioxins and dioxin-like compounds in low temperature thermal processes.

The main findings include the following:

Pressurized liquid extraction (PLE) is applicable for simultaneous extraction of PCDDs, PCDFs, PCNs, polychlorinated phenols and benzenes from thermally treated wood. The choice of solvent for PLE is critical, and the extraction efficiency depends on the degrees of biomass carbonization.

In MAP experiments PCDDs, PCDFs and PCNs were predominantly found in pyrolysis oils, while in torrefaction experiments they were mainly retained in solid chars with minor fractions in volatiles. In both cases, highly chlorinated congeners with low volatility tended to retain on particles whereas the less chlorinated congeners tended to volatize into the gas phase.

Isomer patterns of PCDDs, PCDFs and PCNs generated in MAP were more selective than those reported in combustion processes. The presence of isomers with low thermodynamic stability suggests that the pathway of POPs formation in MAP may be governed not only by thermodynamic stabilities but also by kinetic factors.

Formation of PCDDs, PCDFs and PCNs depends not only on the chlorine contents in biomass but also the presence of metal catalysts and organic/metal-based preservatives.

Overall, the results provide information on the formation characteristics of PCDDs, PCDFs and PCNs in MAP and torrefaction. The obtained knowledge is useful regarding management and utilization of thermally treated biomass with minimum environmental impact.

2

Sammanfattning (Summary in Swedish)

Omställningen till en hållbar energiförsörjning gör att det finns ett ökande

behov att använda biomassa som ersättning för fossila bränslen. Torrefiering

och pyrolys vid måttlig temperatur är förädlingstekniker som har stor

potential för detta ändamål. En viktig aspekt vid införande och

implementering av nya behandlings- och förädlingstekniker är dock att

eventuella miljörisker måste kartläggas och bildning av persistenta organiska

föroreningar (så kallade POPar) såsom dioxiner och dioxinlika föreningar

måste minimeras.

Denna avhandling fokuserar på bildning av polyklorerade dibenso-p-dioxiner

(PCDD), dibensofuraner (PCDF) och naftalener (PCN) i biomassa som

behandlats med mikrovågsassisterad pyrolys respektive torrefiering. Det

huvudsakliga syftet var att studera förekomst och bildning av PCDD, PCDF

och PCN i de produkter som genereras (kol, olja/vattenfas, och gas) med fokus

på s.k. ”fingeravtryck” (homologprofiler och isomermönster) och de

underliggande bildningsmekanismerna. I projektet har även analytiska

metoder för extraktion av klorerade aromatiska föreningar i fasta

pyrolysprodukter utvecklats. Några av de best betydelsefulla resultaten från

avhandlingen är följande:

Extraktion av PCDD, PCDF, PCN, polyklorerade fenoler och bensener

från fasta pyrolysprodukter med hjälp av lösningsmedel vid högt tryck

och hög temperatur är en effektiv extraktionsmetod. Extraktionens

effektivitet är dock beroende av valet av lösningsmedel och graden av

karbonisering av biomassan.

Vid mikrovågsassisterad pyrolys återfanns PCDD, PCDF och PCN

främst i pyrolysoljorna medan de vid torrefiering återfanns

huvudsakligen i de fasta pyrolysprodukterna med bara en mindre

delmängd i oljefasen.

Isomermönstren av PCDD, PCDF och PCN vid mikrovågsassisterad

pyrolys omfattar färre isomerer än de som rapporterats från

exempelvis förbränningsprocesser. Sannolikt kan detta förklaras av

att bildningsvägar som involverar (klor)fenoler och reaktiva

fenoxiradikal-intermediat är mer aktiva i denna process.

Förekomsten av isomerer med låg termodynamisk stabilitet tyder på

att bildningsvägarna POPs i mikrovågsassisterad pyrolys inte bara

regleras av termodynamiska faktorer utan även av kinetiska faktorer

och intermediatens reaktivitet.

Den potentiella bildningen av PCDD, PCDF och PCN beror inte bara

på klorinnehållet i biomassan utan även på förekomsten av

3

katalyserande metaller och organiska och/eller metallbaserade

impregneringsmedel.

Detta avhandlingsarbete har bidragit med kunskap kring bildning av PCDD,

PCDF och PCN vid mikrovågsassisterad pyrolys och torrefiering av biomassa

som kan bidra till produktion och nyttjande av termiskt förädlad biomassa

med minimal miljöpåverkan.

4

List of Abbreviations

CCA Chromated copper arsenate

GC/MS Gas chromatography/Mass spectrometry

DCM

MAP

Dichloromethane

Microwave-assisted pyrolysis

PAH Polycyclic aromatic hydrocarbon

PCBz Polychlorinated benzene

PCDD Polychlorinated dibenzo-p-dioxin

PCDF Polychlorinated dibenzofuran

PCN Polychlorinated naphthalene

PCP Pentachlorophenol

PCPh Polychlorinated phenol

PIC Product of incomplete combustion

PLE Pressurized liquid extraction

POPs Persistent organic pollutants

PUF Polyurethane foam

TEF Toxic equivalency factor

TEQ Toxic equivalent

5

List of Publications

This thesis is based on studies described in the following four appended

papers, which are referred to in the text by the corresponding Roman

numerals.

I. Gao, Q., Haglund, P., Pommer, L., Jansson, S., 2015. Evaluation of

solvent for pressurized liquid extraction of PCDD, PCDF, PCN, PCBz, PCPh and PAH in torrefied woody biomass. Fuel, 154, 52-58.

II. Gao, Q., Budarin, V.L., Cieplik, M., Gronnow, M., Jansson, S., 2015.

PCDDs, PCDFs and PCNs in products of microwave-assisted pyrolysis of woody biomass - Distribution among solid, liquid and gaseous phases and effects of material composition. Chemosphere, 145, 193-199.

III. Gao, Q., Cieplik, M., Budarin, V.L., Gronnow, M., Jansson, S., 2016.

Mechanistic evaluation of polychlorinated, dibenzo-p-dioxin, dibenzofuran and naphthalene isomer fingerprints in microwave pyrolysis of biomass. Chemosphere, 150, 168-175.

IV. Gao, Q., Edo, M., Larsson, S.H., Collina, E., Rudolfsson, M., Gallina M., Jansson, S., 2016. Physical transformation and formation of PCDDs and PCDFs in torrefaction of biomass with different chemical composition. Manuscript

Published papers are reproduced with permission from publisher (Elservier

Science)

6

Author’s contributions

Paper I

I was involved in planning the experiment. I was responsible for the lab work,

including sample cleanup and GC-MS analysis, data evaluation and

interpretation, and writing the paper.

Paper II

I was involved in planning the experiment. I performed the pyrolysis

experiments, in co-operation with co-authors at York University. I was also

responsible for sample treatment, GC/MS analysis, evaluating the data and

writing the paper.

Paper III

I was responsible for evaluating and interpreting data obtained from

experiments of Paper II, and writing the paper.

Paper IV

I planned and performed the experiments in co-operation with my co-authors,

and I was responsible for both evaluating results and writing the paper.

7

1. Introduction

In the transition to sustainable energy supplies, there is an increasing need for

the use of biomass as a replacement of fossil fuel. Utilization of biomass as an

energy carrier can reduce the total atmospheric greenhouse gas burden and

mitigate global warming. An important role in renewable energy supplies has

been assigned to biomass in future energy scenarios and plans (McKendry,

2002). The European directive on promotion of the use of renewable energy

(2009/28/EC) sets goals that, by 2020, at least 20% of the total energy

consumption should be obtained from renewable sources including biomass

(Tanger et al., 2013).

Biomass has several drawbacks as a substitute for fossil fuel, including low

energy density, high moisture content and hydrophilicity (Yan et al., 2009).

Use of untreated biomass is also problematic due to its susceptibility to

microbial degradation, heterogeneous composition and high bulk volume,

which complicate process control and logistics management (Uslu et al.,

2008). In modern practice, biomass conversions are often necessary to

improve properties to reach appropriate characteristics and desirable quality

as fuels.

Among the technologies for biomass thermal conversion, pyrolysis and

torrefaction have becoming increasingly accessible for both pilot and industry

scale. Conventional pyrolysis and torrefaction utilize moderate temperatures

(400–550 °C and 200–350 °C, respectively) at oxygen-deficient conditions

(Demirbaş and Arin, 2002; Van der Stelt et al., 2011). Primary products

include liquid (in pyrolysis) and carbon-rich char (pyrolysis and torrefaction).

The char products provide a number of potential applications, including

energy production through coal co-firing. Biochar from pyrolysis can also be

utilized for soil amendment and long-term carbon sequestration. The

pyrolytic liquid (as a result of condensation of volatiles) can be used as a fuel

product (known as bio-oil) after further upgrading and/or as an intermediate

for synthesis of fine chemicals (Demirbaş and Arin, 2002).

Although thermal conversion of biomass is considered to be a sustainable

alternative, a key challenge in the global transition to renewable energy

supplies is to utilize biomass in an environmentally sound manner. Important

aspects are to minimize the potential formation of persistent organic

pollutants (POPs), such as polychlorinated dibenzo-p-dioxins (PCDDs) and

dibenzofurans (PCDFs) must be minimized.

8

PCDDs and PCDFs, commonly known as dioxins, are by-products of

thermochemical processes and are among the prioritized POPs included in the

Stockholm Convention, a global treaty aimed to protect human health and the

environment (UNEP, 2001). Polychlorinated naphthalenes (PCNs) may also

be formed along with PCDDs and PCDFs, and are considered to be “dioxin-

like” as they have similar chemical structures and toxicological properties

(Schneider et al., 1998). The combination of an inadequate processing

temperature and insufficient oxygen favors formation and survival of

chlorinated aromatics. Studies from waste incineration have shown that, at

low temperature region (200-400 °C) dioxins and other trace chlorinated

organics can be formed via complex heterogeneous reactions catalyzed by fly

ash (Altarawneh et al., 2009). Two main mechanisms have been proposed: the

surface-mediated precursor pathway and the so-called de novo synthesis from

carbonaceous matrix (Born et al., 1993b; Stieglitz, 1998). Chlorine sources (

Cl2, Cl radicals or chlorinated precursors) and metal catalysts appear to be

essential for dioxin formation (Addink and Altwicker, 1998; Procaccini et al.,

2003).

In spite of the fact that the temperatures employed in pyrolysis and

torrefaction are within the range in which chlorinated compounds are formed,

little is known regarding the potential formation of dioxins and dioxin-like

compounds in those processes. It is unclear whether their formation is

affected by fuel type and operating conditions; and how the chlorinated

organics are distributed among the vapor, liquid and solid products. These

aspects are important from a regulation perspective, regarding utilization of

biomass products from thermochemical conversion. Lack of data on

occurrences, profiles and transformation of POPs in biomass conversion

represents a crucial knowledge gap that need to be filled.

The present project involves studies on formation characteristics of POPs

from thermochemical conversion of biomass. Thermochemical processes

underlying in this thesis include microwave-assisted pyrolysis (MAP) (Paper

II and III) and torrefaction (Paper IV). The reason to select MAP and

torrefaction as main focus is that both processes are typically operated at low

temperature regions. POPs specifically addressed in this thesis are PCDDs,

PCDFs and PCNs because all of them are considered to be by-products of

thermal processes. The project also involved the development of analytical

methods (Paper I) to enable extraction and analysis of POPs in thermally

treated biomass.

Objectives

The objectives of the work underlying this thesis were to investigate: the levels

and distributions of PCDDs, PCDFs and PCNs in MAP and torrefaction; effects

9

of biomass properties on formation of PCDDs, PCDFs and PCNs; the

fingerprints (homologue profiles and isomer patterns) of chlorinated

compounds; and the underlying formation pathways in MAP and torrefaction.

The overall aim is to contribute to a better understanding of POPs formation

and transformation in thermal conversion of biomass in moderate conditions,

thereby facilitating the development of environment-friendly conversion

strategies.

Specific aspects addressed include:

Effects of key factors (particularly solvent choice) on the efficiency of

pressurized liquid extraction (PLE) for simultaneous extraction of

various POPs from thermally treated biomass (Paper I).

Formation and distribution of PCDDs, PCDFs and PCNs in biomass MAP

processes (Paper II), focusing on levels and relative abundances of

PCDDs, PCDFs and PCNs in gas, liquid and char products.

Fingerprints (homologue profiles and isomer patterns) of PCDDs, PCDFs

and PCNs in MAP products and the underlying formation pathways

(Paper III).

PCDDs and PCDFs in biomass torrefaction, focusing on origins of dioxins

and their physical transformations (Paper IV).

10

2. Background

2.1. Biomass for energy production Biomass, by definition, includes all biological materials derived from living

organisms. In the context for renewable energy source, it often refers to

lignocellulosic materials such as wood, energy crops, agricultural and forest

residues (Yan et al., 2009). Use of biomass to replace fossil fuel has received

increasing interest because of its potential ability to mitigate both energy crisis

and climate change derived by anthropogenic release of CO2. Currently in

Sweden, biomass contributes about 129 TWh, a fifth of the total energy supply

(ER, 2015). The biomass for energy purpose in Sweden is mainly by-products

and residues (e.g., sawdust and bark) flow in the forest and agricultural

industry (Hoffmann and Weih, 2005). This includes biomass from both

conventional forestry (e.g., pine, spruce and other softwood species) and fast-

growing woody crops with short rotation (3-15 years) such as poplar and Salix.

Waste wood, both untreated and preservative-treated, from construction and

demolition can also be used for energy recovery (Krook et al., 2004).

One of the main concerns to use wood for energy recovery from industrial

residues is the uncertainty of its contamination history and the subsequent

environmental impact. Waste wood has often been treated with preservatives

to protect against microbial and fungal decay (Krook et al., 2008). The most

common ways in Sweden for wood treatment were creosote oils or chromated

copper arsenate (CCA), or a mixture of both until end of 1970s (Sundqvist,

2009). Pentachlorophenol (PCP) were also commonly used in Sweden until

1970s for treatment (dip or pressure-impregnation) of wood products

(Sundqvist, 2009). PCP preservatives were often contaminated by dioxins to

various degrees during manufacture. Due to their persistence, both PCP and

dioxins are still present at substantial concentrations in large proportions of

treated wood that is still in service (Piao et al., 2011). One method to destroy

the dioxins when disposing of such contaminated waste wood, and thus

minimize potential environmental risks, is controlled incineration (Sundqvist,

2009). However, it is often difficult to identify whether preservatives have

been used to treat wood, and if so which preservatives, based only on the

wood’s color and odor (Krook et al., 2004).

Despite the contribution of biomass to sustainable energy supplies, there are

several limitations to its use for energy purposes. Firstly, untreated biomass

has much higher H/C and O/C ratios, and thus lower heating value than

conventional fossil fuels (Vassilev et al., 2010). Secondly, its generally

tenacious, fibrous structure and heterogeneous composition complicate

process design and control. Thirdly, agricultural production of biomass is

11

relatively land intensive and is logistically expensive due to the generally low

energy density of biomass (Lam et al., 2015). a key challenge for biomass

based systems is to develop advanced conversion technology in order to

compete with fossil fuels.

2.2. Thermal decomposition of biomass The major components of plant biomass include cellulose, hemicellulose,

lignin and, to lesser extent, organic extractives and inorganic minerals

(Vassilev et al., 2010). Cellulose, hemicellulose and lignin constitute the

polymeric structure of plant cell walls, and their contents vary substantially

depending on the species and parts of the plants the biomass originated from.

Generally, hardwood contains less lignin than softwood but the cellulose

content is more or less the same (McKendry, 2002).

Hemicellulose, cellulose and lignin decompose in different temperature

ranges: 150-300 °C, 300-400 °C and 280-500 °C, respectively (Basu, 2010).

Decomposition processes of these three main components of biomass are

complex, but dehydration and decarboxylation are the main reactions

involved. The main reaction pathways have been categorized into a few

reaction regimes (Bergman et al., 2005), as shown in Figure 1. These include

an initial dehydration and non-reactive temperature stage, followed by

polymer softening, depolymerization, and carbonization/mass loss stages.

The decomposition processes for each polymer are similar, although they

occur at different temperatures. Hemicelluloses have much more diverse

structures than cellulose and lower thermal stability (or less resistance) due

to their lack of crystallinity. Thermal degradation of hemicellulose can start at

temperatures as low as 200°C (Manuel and Metcalf, 2008). The species

formed in the initial depolymerisation or fragmentation reactions may

undergo additional secondary thermal decomposition reactions to form

volatile products. Alternatively, they may be involved in

condensation/polymerization reactions that result in formation of high molar

mass products like char (Manuel and Metcalf, 2008).

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Figure 1. Decomposition regimes of lignocellulosic materials during thermochemical conversion

(Bergman et al., 2005).

Low temperature dehydration and depolymerization of cellulose are

important processes in both slow pyrolysis and torrefaction. Thermal

treatment of wood at temperatures under 250°C leads to thermal degradation

of its constituents and changes in crystallinity (Lam et al., 2015). The

reduction in the degree of polymerization when heating the cellulose for hours

can be associated with the formation of free radicals, elimination of water,

formation of carbonyl and carboxyl groups, as well as the evolution of carbon

dioxide (Sandberg et al., 2013). Moreover, the presence of small amounts of

inorganic impurities (less than 0.1 %) can considerably alter the pyrolysis and

combustion characteristics of cellulose and promote the formation of

hydroxyl-acetaldehyde via fragmentation or open-ring reactions (Lam et al.,

2015).

13

The main products of the thermal decomposition of hemicelluloses found

include acetic acid, furans, and various mono- and oligo-pentoses (Lam et al.,

2015). The thermal degradation of lignin results in the formation of

monomeric phenols, guaiacols and syringols, formic acid, formaldehyde,

methanol, CO2 and water (Sandberg et al., 2013). Generally, the thermal

degradation of lignin can be described by a competitive mechanism involving

depolymerization and condensation/carbonization reactions. Chain

depolymerization is expected to occur via successive formation of the new

phenolic structure through cleaving the phenolic end structure. The phenolic

α and β-ester types of dimers became reactive at temperatures around 200-

250°C, and are among the main products in lignin pyrolysis (Lam et al., 2015).

Thermal decomposition of lignocellulosic biomass results in formation of,

among other substances, a complex mixture of condensable hydrocarbons,

known as tar. The mixture consists of single- to five-ring aromatics, phenolic

compounds and complex polycyclic aromatic hydrocarbons (PAHs)

(Wolfesberger et al., 2009). The amounts and nature of tar formed largely

depend on the feedstock properties, the presence of catalysts and operating

conditions. At moderate temperatures (about 500 °C), the tar-like products

are highly branched, while at high temperatures (over 700 °C) they tend to be

highly condensed and less oxygenated. Naphthalene, one of the most

abundant light tar components, can act as a precursor for PCN formation via

direct chlorination (Jansson et al., 2008). Other PAHs in tar products can also

act as precursors for formation of chlorinated aromatics, via de novo synthesis

and other pathways (Weber et al., 2001).

2.3. Thermochemical conversion Thermochemical conversion is the controlled heating or oxidation of

feedstocks to generate energy products and/or heat (Demirbaş and Arin,

2002). It covers a range of technologies including combustion,

gasification, pyrolysis and torrefaction (Van der Stelt et al., 2011).

Gasification, pyrolysis and torrefaction are all considered to be processes in

which materials are thermally decomposed in the absence of oxygen, or

significantly less oxygen is present than required for complete combustion

(Tanger et al., 2013). Complete combustion involves the production of heat via

oxidation of carbon- and hydrogen-rich biomass to CO2 and H2O. Gasification

is the exothermic partial oxidation of biomass with operating conditions

optimized for high yields of gas products rich in CO, H2 and CH4. Pyrolysis is

the thermal decomposition of biomass in the absence of oxygen. There are no

clear boundaries between the three processes. For example, pyrolysis can be

considered an incomplete gasification process, and torrefaction as an initial

process of both gasification and pyrolysis.

14

The yields of major products (gas, oil and char) vary depending on the

operating conditions (particularly temperature, residence time and oxygen

supply) (Basu, 2013). Fast heating rates and moderate temperature favor

generation of liquid products (pyrolysis), low temperature and long residence

times primarily result in charcoal (torrefaction), while gases (condensable and

non-condensable) are largely produced at high temperature and heating rate

(gasification). The major differences between combustion, gasification,

pyrolysis and torrefaction, in terms of operating conditions, are illustrated in

Figure 2. The studies underlying this thesis primarily focused on pyrolysis

(Papers II and III) and torrefaction (Paper IV), thus the following

description of conversion technologies mainly addresses these two processes.

Figure 2. Comparison of four biomass thermochemical conversion processes — combustion,

gasification, pyrolysis and torrefaction — showing the major products. Figure from Yin et al.

(2012), with modification by the author of this thesis.

2.3.1. Pyrolysis

Pyrolysis is the thermochemical decomposition of biomass at elevated

temperature in the absence of oxygen. The process is endothermic, and

usually carried out in the temperature range 300-650 °C (Mohan et al., 2006).

The main initial products of pyrolysis are condensable gases (bio-oil or

pyrolysis oil) and solid char. Some of the condensable gas may further

decompose into secondary products including CO2, H2 and CH4. The yields

and chemical composition of pyrolysis products depend on feedstock

15

properties, pyrolysis temperature and heating rate. Pyrolysis can be classified

as slow, fast and flash pyrolysis depending on the heating rate, residence time

and reaction rate (Mohan et al., 2006). In fast pyrolysis, the vapor residence

time is about a few seconds and the primary products are bio-oil and gas. In

slow pyrolysis residence times are longer (up to a few minutes), and the

primary product is char.

In addition to conventional pyrolysis, the integration of microwaves and fast

or flash pyrolysis process, known as microwave-assisted pyrolysis, as a

novel conceptual design for biomass conversion, has become increasingly

accessible at both pilot and industrial scales (Yin, 2012). Unlike conventional

pyrolysis where heat is transferred from the surface towards the core of the

feedstock by conduction and convection (Mohan et al., 2006), in MAP thermal

energy is transferred from electromagnetic energy by “dielectric heating”

(Mushtaq et al., 2014). It is a process of energy conversion rather than heating.

Microwave irradiation generates in-core volumetric heating by directly

coupling microwave energy with exposed biomass (Shuttleworth et al., 2012).

It has been reported that the temperature at which biomass decomposes

during MAP was lower than in conventional pyrolysis (Budarin et al., 2010),

partly due to water evaporation, which both removes considerable amounts of

energy from the active centers (thereby cooling them) and redistributes this

energy throughout the rest of the material (Robinson et al., 2010; Fan et al.,

2013).

2.3.2. Torrefaction

Torrefaction is a thermochemical process in oxygen deficient conditions.

Biomass temperatures during torrefaction are typically between 200°C and

350°C, and its residence time ranges from a few minutes to several hours

(Nordin et al., 2013). The torrefaction process is often operated at ambient

pressure, in an inert atmosphere to avoid oxidation and combustion of the

biomass. It is also known as mild and slow pyrolysis although pyrolysis

processes are usually operated at temperatures above 350°C. The word

torrefaction originates from the French word torréfaction, meaning roasting,

typically of coffee beans, a process similar to torrefaction of biomass, but using

air at a relatively low temperature (Basu, 2013). Torrefaction is initiated with

moisture evaporation, followed by partial devolatilization. Part of the biomass

volatilizes and forms a torrefaction gas, which can be combusted and the heat

can be used for biomass drying and for heating of the torrefaction process

(Bergman et al., 2005). Two torrefaction regimes can be defined, depending

on the processing temperature: light and severe. In light torrefaction, at

temperatures below 240 °C, only hemicellulose is decomposed, while lignin

and cellulose are not affected substantially (Bilgic et al., 2016). In severe

torrefaction (at temperatures exceeding 270 °C), cellulose and lignin start to

16

decompose. One of the fundamental advantages of the torrefaction process is

energy densification (typically by a factor of 1.3) via reduction of the O/C and

H/C ratios of the biomass (Van der Stelt et al., 2011). Typically, in woody

biomass torrefaction, about 70% of the mass is retained as a solid product,

which contains 90% of the energy content of the untreated material. The

remaining 30% of the mass is converted into torrefaction gas, which normally

contains only 10% of the initial energy of the biomass (Van der Stelt et al.,

2011). Torrefied biomass has been demonstrated to be suitable as fuel for

various applications, particularly entrained flow gasification, small-scale

combustion and co-firing in pulverized fired power stations (Bergman et al.,

2005). A higher biomass to coal ratio can be used with torrefied fuels than

with untreated biomass (Basu, 2010).

2.4. Dioxins and dioxin-like compounds

2.4.1. General description

Dioxin is a collective term for PCDDs and PCDFs. They are chlorinated

aromatic hydrocarbons that are similar in structure and chemical properties.

They consist of two benzene rings that are interconnected by either two

oxygen bridges (PCDDs) or one oxygen bridge and a single C-C bond (PCDFs),

as shown in Figure 3. Chlorine substitution can occur at carbon atoms

numbered 1 – 9 in the structures shown in Figure 3, resulting in a total of 75

and 135 individual PCDD and PCDF congeners, respectively. PCDDs or

PCDFs with the same number of chlorine atoms constitute a homologue

group of isomers. The relative abundance of congeners within each

homologue group is referred to as an isomer distribution pattern. There

are 16 homologue groups in total, 8 of PCDDs and 8 of PCDFs (Table 1). For

example, within the pentachlorinated homologue group, there are 10 and 16

different PCDD and PCDF isomers, respectively.

Figure 3. PCDD, PCDF and PCN structures and substitution positions.

17

Some chlorinated compounds have been classified as “dioxin-like” due to

the similarity of their chemical structures, physicochemical properties and

toxic behaviors. These include some non-ortho- or mono-ortho-substituted

(tetra- to hepta-) polychlorinated biphenyls (PCBs). Another emerging group

with dioxin-like properties is PCNs. PCNs consist of two fused benzene rings

that share two carbon atoms. Chlorination can occur at positions 1-8 (Figure

3), resulting in a total of 75 different PCN congeners (Table 1). Only laterally

substituted tetra- to octa-CNs are considered to be dioxin-like.

Table 1. Numbers of isomers in each group of PCDD, PCDF and PCN

homologues

Homologues Number of Cl Number of isomers

PCDD PCDF PCN

Mono- 1 2 4 2

Di- 2 10 16 10

Tri- 3 14 28 14

Tetra- 4 22 38 22

Penta- 5 14 28 14

Hexa- 6 10 16 10

Hepta- 7 2 4 2

Octa- 8 1 1 1

Total 75 135 75

2.4.2. Toxicity of dioxins Dioxins in the environmental media as well as biological samples exist as

mixture of various congener. The toxicity varies substantially among the

different congeners. Of the 75 PCDDs and 135 PCDFs, only those fully

substituted in the lateral (β- or 2,3,7,8-) position are toxic (Van den Berg et

al., 2006). Many congeners are present at substantially higher concentrations

than 2,3,7,8-TeCDD.

To facilitate and simplify risk assessment of potential exposure to dioxins,

various toxicity equivalency factors (TEFs) systems have been developed

to express the relative toxicity. 2,3,7,8-TCDD which is considered to be the

most toxic congener of the PCDD/Fs is assigned a TEF value of 1 and the

remaining congeners with a lower toxicity relative to 2,3,7,8-TeCDD have

lower TEF values than 1 (Van den Berg et al., 2006). The criteria to include a

18

compound in a TEF scheme include a structural relationship to the PCDD/Fs,

binding to the aryl hydrocarbon receptor, persistence and accumulation in the

food-chain. By multiplying the mass or concentration of each dioxin congener

with its corresponding TEF-value, toxic equivalents (TEQs) of each

congener concentration are obtained. The overall toxicity of dioxins in a

sample can then be simply calculated by summing the TEQ for all of the

detected congeners. A number of TEQ/TEF systems have been developed in

recent decades. The two most commonly used are the International TEF (I-

TEF) system which used in the EN 1948 standard (ECS, 2006) and the WHO-

TEQ system developed by the World Health Organization (Van den Berg et al.,

2006). The WHO system is similar to the I-TEF scheme, except for the

inclusion of TEFs for dioxin-like coplanar PCBs and the assessment of

pentachloro- and octachloro-congeners. I-TEF values were used for TEQ

evaluations in the studies underlying this thesis since coplanar PCBs were not

analyzed. TEFs of the 17 toxic PCDD/Fs according to the I-TEF system are

summarized in Table 2.

Although TEF values have not yet been determined for most PCNs, some PCN

congeners have been suggested to be active enzyme inducers and bind to the

aryl hydrocarbon receptor (Falandysz, 1998). Two of these congeners,

1,2,3,4,6,7-HxCN and 1,2,3,5,6,7-HxCN, have been frequently identified in

human and environmental samples, and identified as highly persistent and

bioaccumulating (Hooth et al., 2012). Thus, inclusion of PCNs in the WHO-

TEF scheme has been proposed (Van den Berg et al., 2006).

Table 2. Toxic equivalency factors (I-TEFs) for PCDDs and PCDFs

PCDDs TEF

PCDFs TEF

2,3,7,8-TeCDD 1 2,3,7,8-TeCDF 0.1

1,2,3,7,8-PeCDD 0.5 1,2,3,7,8-PeCDF 0.05

1,2,3,4,7,8-HxCDD 0.1 2,3,4,7,8-PeCDF 0.5

1,2,3,6,7,8- HxCDD 0.1 1,2,3,4,7,8-HxCDF 0.1

1,2,3,7,8,9- HxCDD 0.1 1,2,3,6,7,8- HxCDF 0.1

1,2,3,4,6,7,8-HpCDD 0.01 2,3,4,6,7,8- HxCDF 0.1

OCDD 0.001 1,2,3, 7,8,9- HxCDF 0.1

1,2,3,4,6,7,8- HpCDF 0.01

1,2,3,4, 7,8,9- HpCDF 0.01

OCDF 0.001

19

2.4.3. Formation mechanisms

Thermal formation of PCDD and PCDF can occur both homogeneously (in

gas phase) and heterogeneously (on solid surfaces such as soot or ash). The

homogeneous pathway involves gas phase reactions with precursors at

temperatures of 400-800 °C. PCPh and PCBz are among the most important

precursors (Born et al., 1993). Heterogeneous formation proceeds via two

surface-catalyzed reaction pathways. One is known as de novo synthesis, a

process involving simultaneous oxidation and chlorination from a

carbonaceous matrix at temperatures between 200 and 400 °C (Stieglitz,

1998). The other is the precursor pathway, involving metal catalysts

(mainly Cu and Fe) at temperatures between 200 and 400 °C. The

homogeneous pathway in gas phase is much less important than the two

heterogeneous (de novo synthesis and catalyst-related precursor) pathways.

De novo synthesis is one of the dominant pathways for PCDF formation,

whereas PCDDs often originate from precursors with limited contributions of

de novo pathways (Hell et al., 2001; Wikström et al., 2004).

Precursor pathway involves condensation of structurally-related

precursors, particularly phenolic species. Homogeneous reactions occur via

gas phase condensation of precursors, whereas the heterogeneous pathway

involves reactions promoted by transition metals (mainly Cu and Fe).

Chlorophenols can either be formed via catalyzed reactions or released

directly from the fuel. Chlorobenzenes can also act as precursors. However,

the formation rate from chlorobenzene is much slower (two orders of

magnitude) than formation from corresponding chlorophenols (Ghorishi and

Altwicker, 1996). It is likely that the formation follows the same phenoxy

radical intermediates, which can be formed by oxidation of chlorobenzenes.

The formation pathway from precursors is mainly associated with the

coupling of molecule/molecule, molecule/radical, or radical/radical species.

An important step is formation of the phenoxy radical from a phenol molecule.

Under pyrolytic conditions, formation of chlorophenoxy radicals is mainly

initiated through thermal decomposition of chlorophenol species. O-H bond

fission or cleavage (R1) is believed to be more important than direct expulsion

of a Cl atom, based on calculated rate constants (Evans and Dellinger, 2003):

2-C6H4ClOH 2-C6H4ClO· + H (R1)

When oxygen is present, decomposition of chlorophenol can commence at

temperatures 150 K lower. An oxygen molecule can react initially with

chlorophenol by abstraction of its phenolic H to form HO2:

2-C6H4ClOH + O2 2-C6H4ClO· + HO2 (R2)

20

In the presence of Cl atoms, Cl radical is the dominant abstractor of the

phenolic H, resulting in HCl production:

2-C6H4ClOH + Cl 2-C6H4ClO· + HCl (R3)

Phenoxy radical exhibits low reactivity with oxygen and does not undergo

decomposition. Phenoxy radical has a resonance structure and the radical can

be localized not only on the phenolic oxygen, but also on the para- and ortho-

carbons (Figure 4). Thus, self-condensation of phenoxy radical can occur via

the ortho- and para-carbon and the phenolic oxygen. The major product of

self-coupling is believed to be para-para dimers, based on molecular spin

density of orbital theory (Libit and Hoffmann, 1974). Experimental study

under pyrolysis condition showed that, at low temperature (500-850 k), the

major product of the self-dimerization of the phenoxy radicals was via ortho-

ortho coupling (Wiater et al., 2000). The para-para coupling was less

important due to the relatively high activation energy. Instead, kinetic process

related to activation energy is a dominating factor in terms of the formation of

ortho-ortho coupling (Armstrong et al., 1983). Regarding the self-

condensation of chlorinated phenoxy radicals, pathway via ortho-carbons

substituted with chlorine atoms could be inhibited due to steric effect (Weber

and Hagenmaier, 1999).

Figure 4. Phenoxy intermediates with radical localized at different positons.

The formation of PCDFs is exclusively the result of the condensation of two

radicals, while the formation of PCDDs can involve radical/radical,

molecule/radical or molecule/molecule coupling (Evans and Dellinger,

2003). Among the different type of coupling in PCDD formations, the

radical/radical pathway is kinetically favoured. The chlorination patterns of

chlorophenols for PCDD formation also differ from those required for PCDF

formation. Formation of PCDDs requires both chlorophenols to have at least

one ortho-chlorine, while formation of PCDFs requires both chlorophenols to

21

have one available ortho-hydrogen. This explains the exclusive formation of

PCDDs from 2,3,6-trichlorophenoxy intermediate since there is not ortho-

hydrogen available for PCDF formation (Altarawneh et al., 2009).

De novo synthesis involves oxidative decomposition of a carbonaceous

matrix (e.g., soot, activated carbon or char). Carbon with oxygen incorporated

in the macromolecule yield greater amounts of PCDDs and PCDFs than more

ordered carbon sources such as graphitic matrices (Tame et al., 2007). Two

basic reactions are involved, chlorination and oxidation: inorganic chlorine is

initially transferred to the macromolecular carbonaceous structure and form

C-Cl bond, then oxidative degradation of the structure.

The de novo synthesis pathways should not be considered separately from the

precursor pathway. Rather, the formation of dioxins proceeds through

numerous reactions, some of which can be considered as de novo synthesis by

definition, whereas others involve precursor intermediates. Figure 5 presents

a schematic of de novo formation of PCDDs and PCDFs (Tame et al., 2007)

showing a number of routes involved in de novo processes. For example, the

de novo pathway can be initiated by oxidation of a carbon matrix to form CO,

CO2, short chain molecules, benzene and macromolecules. The formation of

PCDDs and PCDFs involves: i) precursor molecules such as (chloro)phenol

intermediates; ii) direct formation from surface-bound chlorinated or non-

chlorinated aromatic intermediates such as PAHs or phenolic compounds;

and iii) direct release of pre-existing PCDD and PCDF structures during

oxidation, when oxygen is incorporated in the carbon skeleton). The de novo

pathway occurs primarily in the 200-400 °C temperature range. Oxygen is

necessary for de novo formation of PCDDs and PCDFs, and their formation is

most favourable with O2 concentrations around 5-10% in the reactant stream

(Addink and Olie, 1995).

22

Figure 5. Schematic diagram of de novo formation of PCDDs and PCDFs. The red arrows

indicate dominant routes (Tame et al., 2007).

Chlorination/dechlorination mechanisms have been investigated

extensively. The chlorine in the feedstock can be released mainly (>90%) as

HCl. HCl is normally first oxidized to Cl2 gas, and subsequently chlorinate

aromatics, rather than acting directly as a chlorination agent. The Deacon

process is believed to be important for conversion of HCl to Cl2 in the presence

of O2 and a metal catalyst (Hisham and Benson, 1995). Copper chloride

species (CuCl and CuCl2) are the most important Deacon catalysts with

divalent copper chloride being most efficient. The overall reaction can be

summarized as:

HCl + 1/4O2 ½ H2O + ½ Cl2 (R4)

The process involves two steps as shown in R5 and R6. The Cl2 formed in the

reactions can subsequently participate in chlorination of aromatics. It has

23

been shown that the Cl2 yield during the Deacon process was maximal at 400

°C, while at 300 °C the yield was close to zero (Gullett et al., 1990).

CuCl2 + ½ O2 CuO + Cl2 (R5)

CuO + 2 HCl CuCl2+ H2O (R6)

The Cl2 yield from HCl conversion via the Deacon reaction depends on oxygen

content. The Cl2 yield is positively correlated to the O2 concentration in the

range of 0-3% (Gullett et al., 1990). The Deacon reaction is favored at

temperatures higher than 600 °C in the presence of high concentrations of

HCl (Liu et al., 2000). Interestingly, the presence of water vapor in the

reactant stream can shift the Deacon reaction equilibrium to formation of HCl,

thereby suppressing Cl2 liberation (Wikström et al., 2003a). Thus, the

chlorination capacity is reduced and the homologue profile can be shifted

towards more lightly chlorinated groups. It should be noted that the

importance of the Deacon reaction in the formation of dioxins has been

questioned, and an alternative chlorination route involving aromatic

compounds via reaction with CuCl2 has been suggested (Born et al., 1993a).

Dechlorination of PCDDs and PCDFs requires the presence of a catalyst of

copper or other metal compound (Weber et al., 2002). The most important

parameters in determining the rate of degradation were the reaction time and

temperature, but at relatively low temperatures, the temperature had a greater

influence than the reaction time. Longer reaction times and higher

temperatures both increased the degradation efficiency (Song et al., 2008).

PCN formation mechanisms have not been studied as extensively as those

of PCDDs and PCDFs. Some studies suggested that PCNs can be formed via

both de novo mechanism from carbon matrices and hydrocarbons, similar as

PCDDs and PCDFs (Ryu et al., 2013). Precursor pathways involving

chlorinated compounds have also been reported, and chlorination of non- and

less chlorinated aromatics is believed to be important in PCN formation (Ryu

et al., 2013). In addition, it has been proposed that the formation of PCNs

correlates with that of PCDFs (Oh et al., 2007). The further chlorination of low

chlorinated compounds is believed to be important in PCN formation

(Jansson et al., 2008).

24

3. Materials and methods

Formation of POPs depends on feedstock properties and operating conditions

of thermal conversions. The presence of Cl and transition metals are among

the crucial factors. To study the effect of biomass composition on the

formation of PCDDs, PCDFs and PCNs, various biomass feedstocks

containing different amounts of Cl and Cu and with different contamination

profiles was chosen in the experiment. Two types of technologies, MAP and

torrefaction were selected to represent thermal conversion at moderate

condition. MAP has a high heating rate and short reaction time; while

torrefaction has a low heating rate but with similar temperature. Both are

under oxygen deficient condition.

3.1. Feedstocks Five biomass feedstocks (Figure 6) were selected for the studies: a very clean

conifer (Norway spruce and Scots pine) stemwood assortment; Norway spruce

bark with higher Cl and Cu contents than the stemwood; impregnated

stemwood (probably treated with organic and metal-based preservatives)

from a discarded telephone pole containing high amounts of heavy metals

(35.9 and 362 mg kg-1 of Cr and As, respectively); cassava stems with high Cl

contents; and particle board containing PCP preservatives. Information on the

five feedstocks (including their Cl, Cu and Fe contents) is summarized in Table

3. More details about their chemical properties and energy contents are

presented in the paper II-IV. The first three feedstocks (stemwood, bark and

impregnated stemwood) were used in both MAP (Papers II and III) and

torrefaction (Paper IV). The cassava stems and particle board were used only

in torrefaction (Paper IV). The biomass used in Paper I to evaluate the PLE

method included torrefied Eucalyptus and spruce wood chips, the chemical

compositions of which were not analyzed.

The term of stemwood used in this thesis was also referred to as softwood in

Paper II and III, since it was from a softwood species. However, for

consistency in terminology, the term stemwood is constantly used in the

thesis.

25

Figure 6. Feedstocks used in MAP experiments (A, B and C) and torrefaction experiments (A to D): stemwood pellets (A), bark pellets (B), impregnated stemwood (C), cassava stems (D), and particle board (E).

26

Table 3. Selected information on feedstocks used in experiments reported in

Papers I-IV.

Feedstocks

Chemical composition

Description

Cl, % Cu,

mg kg-1 Fe, % Ash, %

Stemwood

(Papers II-IV) <0.01 0.97 0.001 0.4

A clean assortment (pellets) made from a mixture of bark-free Norway spruce and Scots pine

Bark

(Papers II-IV) 0.02 3.39 0.04 3.9

Norway spruce bark pellets

Impregnated stemwood

(Papers II-IV) <0.01 3.38 0.006 0.5

Wood chips from a discarded Scots pine telephone pole, presumably impregnated with organic and metal-based preservatives

Cassava stems

(Paper IV) 0.29 3.56 0.006 3.8

Cassava stem pellets made from stem residues after cropping the starchy roots

Particle board

(Paper IV) 0.15 11.4 0.07 2.7

Composed of wood chips, sawmill shavings and sawdust with synthetic resin or other binder, collected at a waste disposal site

Eucalyptus chips

(Paper I) - - - -

Torrefied at 300 °C for 16 min

Spruce wood

(Paper I)

Spruce chips treated by PCP impregnation followed by torrefaction (270 °C for 50 min)

27

3.2. Experimental setup of microwave-assisted pyrolysis and torrefaction MAP and torrefaction were performed at York University (UK) and Swedish

University of Agricultural Sciences (SLU, Umeå), respectively. Both

experiments were conducted at bench-scale (Figure 7 and 8).

The sampling setups both consisted of condensers and gas samplers. For the

MAP experiments there were two condensers connected in series with

different cooling temperatures (0 °C and room temperature), enabling

fractionation of liquid products into oil and aqueous phases during sampling.

In torrefaction experiments the liquid products were collected by a single

condenser with no fractionation of aqueous and oil phases. In both cases, the

non-condensable vapor (gas products) that escaped from the cooling traps

was collected on a glass fiber filter followed by a polyurethane foam plug

(PUF). Vacuum units were connected to the outlet of the gas samplers to

facilitate continuous extraction of volatile products. For torrefaction, nitrogen

was applied to the reactor to ensure an oxygen-deficient atmosphere. All

experiments were run in triplicate, and duplicate field blanks were prepared.

28

Figure 7. MAP experimental setup and schematic diagram of sample collection equipment: 1- Controller, 2- Microwave reactor, 3- First condenser, 4- Pyrolysis liquid collector, 5- Second condenser, 6- Pyrolysis liquid collector, 7- Gas sampling point, 8- Electromagnetic valve, 9- Vacuum pump.

29

Figure 8. Torrefaction experimental setup and schematic diagram of sample collection equipment: 1- Furnace, 2- Tubular reactor, 3- Condenser, 4- Condensables collector, 5- Gas

sampling point, 6- Electromagnetic valve, 7- Vacuum pump.

30

3.3. Sample extraction and cleanup PCDDs, PCDFs and PCNs in gas, liquid and char products were extracted by

Soxhlet extraction, liquid-liquid extraction and PLE, respectively, using

toluene as the extraction solvent. Prior to extraction, 13C-labelled internal

standards were added to the samples. PLE was performed at 160 °C using

three extraction cycles, a flush volume of 60%, 5 min static time and 90 s purge

time.

After extraction samples were cleaned-up by multilayer silica column, then

fractionated using an AX21 carbon/celite column (Figure 9). Corresponding

recovery standards were added to the final solution before gas

chromatography/high resolution mass spectrometry (GC/MS) analysis

(Liljelind et al., 2003). 13C-labeled internal standards were added to samples

before extraction, and recovery standards were added to the final extracts

before instrumental analysis.

3.4. Instrumental analysis PCDDs, PCDFs and PCNs were analyzed by GC/MS, using a Hewlett-Packard

5890 gas chromatograph (Agilent) coupled to an Autospec Ultima mass

spectrometer, equipped with a J&W DB5-ms fused silica capillary column (60

m × 0.25 mm i.d. × 0.25 µm film thickness). For the isomer-specific analysis

in Paper III, samples were re-injected onto a SP 2331 column (60 m × 0.25

mm i. d. × 0.25 µm film thickness) to resolve isomers co-eluting on the DB5-

ms column. Separation of PCN isomers was performed on a J&W fused silica

capillary column DB5 (60 m × 0.25 mm i. d. × 0.25 µm film thickness). Details

of GC/MS parameters for analyzing PCPh, PCBz and PAHs are described in

Paper I.

The analytes were quantified using the isotope dilution method. Field and

laboratory blanks were treated in the same manner as the samples. Data with

recoveries outside the acceptable range of the EN 1948 standard method (ECS,

2006) were excluded.

31

Figure 9. Experimental procedures applied to determine PCDDs, PCDFs and PCNs from

pyrolysis and torrefaction.

Liquid Char

Liquid extraction extraction

Papers II &III Paper IV

PLE Soxhlet

Microwave-assisted pyrolysis Torrefaction

Stemwood Bark Cassava stems

Particle board

Impregnated stemwood

Gas

Extracts

Multilayer silica column

Carbon/Celite column

GC/HRMS

32

4. Results and discussion

4.1. Solvent effects in pressurized liquid extraction This section outlines findings from the analytical methodology evaluation, in

which assessed the applicability of PLE for simultaneous extraction of PCDDs,

PCDFs, PCN, and related precursors from thermally treated biomass (Paper

I). The optimized PLE method developed in the study was subsequently

applied to extract chlorinated compounds in char products obtained via MAP

or torrefaction in Paper II-IV.

Quantitative analysis of multiple chlorinated organics in thermally treated

biomass is challenging due to their ultra-trace levels and the highly complex

matrix. Extractions of target compounds often involve Soxhlet extraction,

which is often time- and labor- intensive, and requires large amounts of

organic solvents. PLE involves extraction with organic solvent at elevated

temperature and pressure, enabling exhaustive extraction of target

compounds with relatively short extraction time and low solvent

consumption. However, as with all other extraction techniques, the extraction

efficiency of PLE is matrix-dependent and the reported methods for

environmental samples (e.g., soils and fly ash) cannot be directly applied to

thermally treated biomass.

The PLE method development was divided into three stages: screening,

optimization and validation, as illustrated in Figure 10. In the screening

stage, a series of PLE experiments was performed to evaluate the performance

of five solvents with different polarities: n-hexane, toluene, dichloromethane

(DCM), methanol and acetone. The extraction efficiency of each solvent was

examined by comparing the recoveries of spiked internal standards and the

contents of co-extracted matrix material. In the optimization stage, the two

best solvents identified during screening were used as a binary solvent

mixture, and its performance in PLE was compared to that of the individual

solvents. In the validation stage, PLE with the best individual solvent and the

binary mixture was further evaluated. The PLE method was also compared

with Soxhlet extraction. The entire experimental design is illustrated in Figure

10.

33

Figure 10. Schematic diagram of experimental design and results of PLE method development (Paper I)

PLE • Methanol • Acetone

• Dichloromethane

• Toluene

• n-Hexane

• Toluene --> Satisfied recoveries

• n-Hexane --> Satisfied recoveries

• Methanol --> Matrix effect • Acetone --> Matrix effect • Dichloromethane --> poor reproducibility

Open column cleanup

GC/HRMS

Untreated eucalyptus

Torrefied Eucalyptus

Open column cleanup

GC/HRMS

Untreated eucalyptus

Torrefied Eucalyptus

PLE • Toluene

• Toluene/n-Hexane (1:1)

Solvent screening

• Toluene --> Satisfied • Toluene/n-Hexane --> Satisfied

Optimization /Binary solvent Method Validation

PCP-impregnated spruce chips (untreated)

Open column cleanup

GC/HRMS

PCP-impregnated spruce chips (torrefied)

• Toluene --> Best performance

• Comparable results by PLE with those of Soxhlet extraction

PLE • Toluene

• Toluene/n-Hexane (1:1)

Soxhlet extraction

34

Solvent screening was conducted by evaluating recoveries of spiked

internal standards using five solvents with different polarities: methanol,

acetone, DCM, toluene and n-hexane. As shown in Figure 11, recoveries from

torrefied wood were poor when using solvents with high polarity such as

methanol and acetone. Extraction of untreated wood was less solvent-

dependent and comparable results were obtained with polar and non-polar

solvents. Toluene provided the best performance of the five investigated

solvents. Amounts of co-extractives obtained indicated that the poor

recoveries from torrefied biomass provided by polar solvents were related to

matrix effects. For example, methanol gave rise to about 6-fold more co-

extractives from torrefied samples than toluene (ca. 1.92 and 0.32% of the

sample mass, respectively).

The high co-extractive yields are probably due to co-extraction of thermally

degraded lignocellulosic compounds. The degraded materials that are rich in

hydroxyl groups may be more soluble in polar solvents with high hydrogen

bonding capability (e.g., acetone and methanol), in accordance with the “like

dissolves like” principle. The consequently higher contents of co-extractives in

polar solvent extracts may interfere with subsequent analyses. Indeed, we

observed that residues of methanol extracts following solvent removal were

dense, highly viscous, gelatinous and (hence) difficult to re-dissolve and

transfer to the cleanup columns. Clearly, some of the analytes may be

encapsulated in such residues, leading to losses of target compounds.

The performance of a solvent mixture was evaluated after the solvent

screening. As the screening experiments revealed that polar solvents with

hydrogen-bonding potential extracted large quantities of interfering

materials, binary mixtures featuring acetone were not investigated. Instead, a

mixture of n-hexane and toluene (1:1) was tested in the hope of achieving

similar extraction efficiency to that for toluene alone while releasing less co-

extracted material than when using either of the individual solvents. The

results showed that combination of toluene and n-hexane (1:1) gave

comparable recoveries as those by toluene; while the co-extractive content of

the solvent mixture from the torrefied wood sample was lower (0.24%) than

that achieved using toluene alone.

Method validation was conducted using PCP-impregnated spruce wood

(both untreated and torrefied). Technical PCP normally contains relatively

large quantities of impurities including PCDDs and PCDFs, and has been

widely used as a wood preservative. Impregnation of spruce chips with

technical PCP were aiming to ensure that the torrefied material would contain

measurable quantities of PCDDs and PCDFs so that the developed PLE

35

method could be validated against low, intermediate and high levels of the

target analytes in a single sample.

Comparable results were obtained using toluene and the binary mixture for

most of the PCDDs and PCDFs. However, toluene provided higher yields of

some PCNs, PAHs, and most of the PCBzs and PCPhs in the torrefied wood

samples. The difference in performance between toluene and the binary

mixture was less pronounced for the untreated wood sample. This indicates

that analyte-matrix interactions are stronger in torrefied wood than in

untreated wood, and that toluene disrupts these interactions more efficiently

than the binary mixture. The comparison of PLE and Soxhlet results showed

that the PLE method with toluene provided similar performance to traditional

Soxhlet methodology for extracting PCDDs and PCDFs, and better

performance for extracting PCNs, PAHs, PCBz and PCPh.

In summary, our results indicated that the choice of solvent for PLE is critical

because the extraction efficiency depends on the nature of the biomass matrix

as well as properties of the target analytes. Solvents with high polarity release

high amounts of interfering co-extractives from thermally degraded

hemicellulostic biomass, while non-polar solvents such as hexane do not

efficiently extract the target analytes from torrefied wood.

36

Figure 11. Recoveries of PCDD, PCDF, PCN, PAH, PCBz and PCPh from untreated (A) and

torrefied wood (B) using five indicated extraction solvents (Paper I). Error bars represent ± 1

standard deviations.

0

0,2

0,4

0,6

0,8

1

1,2

Hexane Toluene DCM Acetone Methanol

A

PCDD PCDF PCN

0

0,2

0,4

0,6

0,8

1

1,2

Hexane Toluene DCM Acetone Methanol

B

PCDD PCDF PCN PAH PCBz PCPh

37

4.2. PCDDs, PCDFs and PCNs in microwave pyrolysis

This subsection provides findings from MAP studies (Paper II and III).

Paper II reports levels and relative distributions of the chlorinated

compounds detected among gaseous, liquid and char products. In Paper III,

we investigated the homologue profiles and isomer patterns in MAP products,

and proposed possible formation pathways in MAP based on fingerprints of

the products (Paper III).

As mentioned above, three feedstocks were used in MAP experiments:

stemwood, bark and impregnated stemwood. The resulting non-condensable

gases, liquids (oil and aqueous phases) and chars were collected and analyzed

for PCDDs, PCDFs and PCNs. The entire MAP process took around 10 min,

including heating up time. The temperature profile over the course of the MAP

process varied slightly with the feedstock used (Figure 12). Pyrolysis of the

three feedstocks under these conditions yielded liquid fractions amounting to

34-40% of the original feedstock mass, in accordance with results of previous

MAP studies using wood and agricultural biomass as feedstocks at comparable

temperatures (Budarin et al., 2009; Robinson et al., 2010).

Figure 12. Temperature (T) and pressure profiles during MAP treatment of the indicated

feedstocks (Paper II).

0

100

200

300

400

500

600

700

0

50

100

150

200

250

1 3 5 7 9

Pre

ssu

re,

mb

ar

Tem

per

atu

re, °C

Time, min

T (stemwood) T (bark)

T (impregnated stemwood) P(stemwood)

P(bark) P (impregnated stemwood)

38

4.2.1. Levels and relative distributions

Concentrations of PCDDs, PCDFs and PCNs in the MAP products varied

depending on the feedstock, phase of products (gas, liquid and char) and

compound groups (Figure 13). The highest concentrations of investigated

compounds were found in bark products – about three times higher than

those found in impregnated stemwood products. The lowest concentrations

were found in stemwood products. The high concentrations in bark pyrolysis

products could be related to the relatively high contents of chlorine and

transition metals in the feedstock. In addition, the generally high contents of

phenolic extractives in tree bark and organic contaminants originating from

atmospheric deposition could also lead to formation of chlorinated organics,

as discussed below. The pyrolysis of impregnated stemwood yielded higher

concentrations of PCDDs, PCDFs and PCNs than pyrolysis of clean stemwood.

This could potentially be associated with the presence of metal-based

preservatives, which could sustain smoldering during pyrolysis. The presence

of transition metals could also contribute to the higher concentrations of

analytes, although the Cl content of the impregnated stemwood was similar to

that of the clean stemwood. The effect of biomass chemical composition on

the formation of chlorinated compounds is discussed further in the following

text, together with the results obtained from torrefaction experiments (Paper

IV).

PCDDs, PCDFs and PCNs were most abundant in the oil fractions of the

pyrolysis products, which contained up to 68% of the total measured outputs

of these analytes in each MAP experiment. The relative abundances of these

compounds in the non-condensable gas and aqueous phases was much lower,

possibly due to a “wet scrubbing” effect, i.e. physical or chemical trapping of

gases in droplets of the liquid products (Zwart et al., 2009). The partitioning

of the bulk of the POP content into the oil fraction may be due to its

hydrophobicity and the distillation effect, together with wet scrubbing as

noted above.

The distribution of PCNs among the product fractions differed slightly from

the PCDD and PCDF distributions. For example, the relative abundances of

PCNs in char products of both bark and impregnated stemwood (32.4 and

23.9%, respectively) were substantially greater than those of PCDDs or

PCDFs, which were quite similar (26.7-28.9% in bark products and 16.0-

18.9% in impregnated wood products). This could be related to differences in

absorptive interactions of these compound classes with the carbon matrix.

39

Figure 13. Concentrations and distributions of PCDDs, PCDFs and PCNs in MAP products of stemwood, bark and impregnated stemwood (ng kg -1). Error bars represent ± 1 SD (Paper II).

0

35

70

Stemwood Bark Impregnated stemwood

ng

/kg

in

pu

t

PCDDs

0

40

80

Stemwood Bark Impregnated stemwood

ng

/kg

in

pu

t

PCDFs

0

35

70

Stemwood Bark Impregnated stemwood

ng

/kg

in

pu

t

PCNs

Gas Oil Aqueous Char

40

4.2.2. Homologue profiles

The evaluation of the homologue profiles primarily focused on the oil and char

products since they accounted for most of the PCDDs, PCDFs and PCNs (83±

9%). The homologue profiles (Figure 14) of all three compound groups were

dominated by the less chlorinated homologues. The homologue abundance

decreased as the degree of chlorination increased. This trend was observed for

all three feedstocks, and is consistent with results of earlier pyrolysis studies

(Weber and Sakurai, 2001). The oxygen-deficient conditions used in pyrolysis

are believed to be responsible for the trend because they probably limit the

formation of free chlorine via Deacon-type metal-catalyzed reactions. The

importance of oxygen deficiency was demonstrated by Pekarek et al. (Pekárek

et al., 2001), who observed a dramatic shift towards less chlorinated PCDDs

and PCDFs in pyrolysis products as the oxygen content was reduced from 10%

to < 0.001 %. We postulated that low process temperatures could also restrict

chlorination degrees, since chlorination is highly temperature-dependent

(Jansson et al., 2008).

In bark and impregnated stemwood MAP products, the oil fractions appeared

to have greater relative abundances of less chlorinated homologues than the

chars (Figure 14). The observed differences in homologue profiles between the

oil and char products could be related to difference in vapor pressures

between the low and high chlorinated congeners. The highly chlorinated

congeners with low volatility tend to be retained by particles whereas the less

chlorinated congeners tend to volatilize into the gas phase.

41

Figure 14. Degree of chlorination (#Cl) and relative abundance of PCDD (mono-to octa-), PCDF

(mono-to octa-) and PCN (tri-to octa-) homologues in pyrolysis oils and chars. Error bars

represent ±1 SD (Paper II).

0

1

2

3

4

5

0%

20%

40%

60%

80%

100%

Oil Char Oil Char Oil Char

Stemwood Bark Impregnatedstemwood

De

gr

ee

of

ch

lor

ina

tio

n

Re

lati

ve

% o

f P

CD

D

0

1

2

3

4

5

0%

20%

40%

60%

80%

100%

Oil Char Oil Char Oil Char

Stemwood Bark Impregnatedstemwood

De

gr

ee

of

ch

lor

ina

tio

n

Re

lati

ve

% o

f P

CD

F

0

1

2

3

4

5

0%

20%

40%

60%

80%

100%

Oil Char Oil Char Oil Char

Stemwood Bark Impregnatedstemwood

De

gr

ee

of

ch

lor

ina

tio

n

Re

lati

ve

% o

f P

CN

Mono- Di- Tri- Te- Pe-

Hx- Hp- Octa- # Cl

42

4.2.3. Isomer patterns and formation pathways

This subsection provides findings of Paper III on isomer patterns of PCDDs,

PCDFs and PCNs in MAP, and inferences regarding formation pathways. The

discussion mainly focuses on low chlorinated homologues (mono- to tetra-),

due to the low detected concentrations of penta- to octa-chlorinated

homologues (some are close to detection limit), which hindered identification

of clear isomer patterns. The results of isomers in stemwood MAP products

are not discussed due to their low concentrations.

PCDF isomer distributions (Figure 15) in bark and impregnated stemwood

MAP products were similar, both being dominated by 2-MoCDF, 4-MoCDF,

2,4-DiCDF, 2,4,6-TriCDF, 2,4,8-TriCDF and 1,2,4,6-TeCDF. The isomer

patterns were also less complex than those reported in wood burning and

waste incineration products (Bacher et al., 1992; Wikström and Marklund,

2000), which usually consisted of many isomers at comparable

concentrations. The MoCDF signature in MAP products, with dominance of

4- and 2-MoCDF, differed from signatures frequently reported in combustion

processes, where 2-MoCDF and 3-MoCDF were generally dominating (Ryu et

al., 2004; Ryu et al., 2006). The dominating 2-MoCDF and 3-MoCDF from

combustion processes has been reported to be mainly a result of chlorination

from unsubstituted DF (Wikström and Marklund, 2000). The differences

between the MoCDF patterns suggest that initial chlorination directly from

DF could be less prevalent in MAP. The temperature employed in our MAP

study (up to 200 °C) is probably insufficient to support extensive formation of

DF, for which a high temperature (>650 °C) is often required for condensation

of unsubstituted phenol and benzene (Wikström and Marklund, 2000).

Dominant formation of 2- and 4-MoCDF has been previously reported, in

both pyrolysis and oxidation-driven experiments, using various chlorinated

and unsubstituted phenols as precursor compounds (Yang et al., 1998; Ryu et

al., 2005). Reactions involving (chloro)phenoxyl radicals as precursors have

been proposed to be essential for the formation of particular isomers. The

relative reaction rate involving formation of radical intermediates is one of the

main factors governing the patterns of isomer distributions (Evans and

Dellinger, 2003), as further discussed in the following text.

43

Figure 15. PCDF isomer distributions of MAP products of bark and impregnated wood. The

results are presented as total concentrations (sum of gas, liquid and char) after subtraction of

the concentrations of untreated feedstocks. Error bars represent ± 1 standard deviation (n=3)

(Paper III).

0,0

0,1

0,2

12

368

/

12

478

/

14

678

-

13

479

-

12

346

-

12

378

-

12

367

/

12

678

-

23

467

-

23

478

-

12

389

-

0

4

8

0

6

12

1- 3- 2- 4-

0

3

6

0

2

4

0,0

0,1

0,2

123

46

8-

124

67

8/

134

67

8-

124

68

9-

123

46

7-

123

47

8-

123

67

8-

23

46

78

/12

36

89

-

123

48

9/

123

78

9-

0,0

0,2

0,4

1234679- 1234689-

Bark Impregnated

Co

nc

en

tra

tio

n,

ng

kg

-1

44

PCDD isomer distributions (Figure 16) were dominated by 1-MoCDD and 2-

MoCDD, 1,3-, 2,7-, 2,8-DiCDD, 137/138-, 136- and 124-TriCDD in both bark

and impregnated stemwood products. TeCDD was exclusively comprised by

1,3,6,8- and 1,3,7,9-TeCDD, which are known to be condensation products of

chlorinated phenol precursors, typically originating from dimerization of

2,4,6-chlorophenol (Sidhu et al., 1995). Apart from their formation in

combustion processes, such a reaction has been demonstrated to take place

readily at low temperature (< 300 °C) via metal-mediated catalysis (Manuel

and Metcalf, 2008).

The differences in patterns of MoCDD congeners observed in MAP and

combustion products are particularly notable. 1- and 2-MoCDD were present

at similar concentrations in MAP products, while 2-MoCDD is substantially

less prevalent in wood burning and waste incineration products. 1-MoCDD is

much less thermodynamically stable than 2-MoCDD (Wang et al., 2004). The

higher proportion of 1-MoCDD in the MAP products may be due to the

temperature and residence time employed in MAP being too low to reach

thermodynamic equilibrium.

45

Figure 16. PCDD isomer distributions in MAP products of bark and impregnated wood. The

results are presented as total concentrations after subtraction from concentrations in

feedstocks. Error bars represent ± 1 standard deviation (n=3) (Paper III)

0

1

12468/

12479-

12469- 12368- 12478- 12379- 12369/

12476-

12346/

12347-

12378/

12367-

12389-

0,00

2,00

4,00

2- 1-

0

3

6

1,3- 2,7- 2,3- 2,8-

0,0

1,0

2,0

137/138- 136- 124- 147/237-

0

2

4

1368- 1379- 1248/1246/

1247/1249-

1378- 1269/1234-

0

1

2

123468- 123679/

123689-

123678- 123467- 123789-

0

1

2

1234679- 1234678-

Bark Impregnated

Co

nc

en

tra

tio

n,

ng

kg

-1

46

PCN isomer distributions (Figure 17) slightly differed between bark and

impregnated stemwood MAP products. 2,3-, 1,3-DiCN and 1,2,3-TriCN were

major isomers in bark MAP products, while 15/27-, 14/16-DiCN, 1,4,6-, 1,2,7-

CN dominated in impregnated stemwood products. PCN isomer patterns in

bark and impregnated wood both revealed a tendency for sequential chlorine

substitution from DiCN to TriCN.

A similar successive chlorination pattern has previously been observed in a

naphthalene chlorination experiment, in which 1-MoCN, 1,4-DiCN, 1,4,6-

TriCN, and 1,2,4,6-TeCN were found to be the dominant isomers (Ryu et al.,

2013). Chlorination from unsubstituted naphthalene is an important PCN

formation pathway in municipal solid waste combustion (Jansson et al.,

2008). However, the isomer distributions in the combustion study were more

complex than those we observed, suggesting more complex formation

pathways occur during high temperature and oxidative processes. For

example, de novo synthesis or PAH breakdown, common PCN formation

pathways in combustion, are oxidation-driven. The oxygen-deficient

conditions in MAP could limit de novo synthesis reactions. The low process

temperatures could also restrict PCN chlorination to higher degrees, since

PCN chlorination is highly temperature-dependent (Jansson et al., 2008).

Furthermore, the short reaction time and high heating rate in MAP could also

contribute to the observed selective isomer patterns. The relative importance

of these factors for determining isomer patterns in MAP products requires

further investigation.

Another notable aspect of the findings is that the dominant isomer 1,2,3-

TriCN found in bark MAP products is reportedly thermodynamically

unfavorable (Zhai and Wang, 2005). Isomers with higher thermodynamic

stability, substituted at 2,3,6,7-sites that are commonly favored during waste

incineration, were not observed in our study. It appears that successive

chlorination, providing high electrophilic stability, appears to be more

important than the thermodynamic stability or steric hindrance effect (Zhai

and Wang, 2005).

47

Figure 17. PCN isomer distributions in bark and impregnated wood MAP products. The results

are presented as total concentrations (sum of gas, liquid and char) after subtraction of

concentration in feedstock. Error bars represent ± 1 standard deviation (n=3) (Paper III).

0

7

14

136- 135- 137- 146- 124- 125- 126- 127- 167- 123- 145-

0

2

4

13

57-

12

46/1

247

/ 1

257

-

14

67-

13

68/

12

56-

12

35/

13

58-

12

37/1

234

/ 1

267

-

12

45-

12

48-

12

58/

12

68-

14

58-

12

78-

0

10

20

13- 14/16- 15/27- 26/17- 23-

0

1

2

12

357

/

12

467

-

12

457

-

12

468

-

12

346

-

12

356

-

12

367

-

12

456

-

12

478

-

12

358

/

12

368

-

12

458

-

12

345

-

0,0

0,4

0,8

12

346

7/

12

356

7-

12

345

7-

12

356

8-

12

456

8/

12

457

8-

12

357

8-

12

345

6-

0,0

0,2

0,4

1234567- 1234568-

Bark Impregnated

Co

nc

en

tra

tio

n,

ng

kg

-1

48

In summary, the highly selective isomer patterns of PCDDs, PCDFs and PCNs

in MAP products suggest that multiple pathways involving (for instance)

formation and thermal degradation, are probably less involved in MAP than

in combustion processes. For example, de novo synthesis and dechlorination

of highly chlorinated compounds, which typically generate a “combustion”

pattern with no dominance of particular isomers, could be less important in

MAP. Incorporation of oxygen as well as a sufficiently high temperature,

which are essential for de novo formation, might be limiting factors in MAP at

low temperature (up to 200 °C).

The presence of 1,2,3-TriCN, 1-MoCDD and 4-MoCDF, all of which have low

thermodynamic stability, in the products is intriguing. We proposed that

kinetic factors rather than thermodynamic stability may be important in the

MAP process. Reactions involving (chloro)phenoxyl radicals as precursors at

low activation energy could be essential for the formation of particular

isomers (Evans and Dellinger, 2003). We proposed a PCDF formation

pathway involving cross-coupling of (chloro)phenoxyl radical intermediates

and successive chlorination, as shown in Scheme 1. Following initiated

formation of 4-MoCDF by cross-coupling of a phenoxyl and 2-MoCPh radical,

chlorine substitution is directed to the same benzene ring at the 2-position,

providing high electrophilic stability. Subsequent chlorine substitution may

then occur governed by electron density distribution.

49

Scheme 1. Postulated PCDF formation pathways involving cross-coupling of (chlorinated) phenoxyl radical intermediates (Paper III) followed by step-wise chlorination.

4.3. Dioxins in torrefaction products

This section presents findings of the torrefaction study (Paper IV). The

temperature profile observed during the course of torrefaction of each type of

feedstock used is shown in Figure 18. The entire torrefaction process,

including heating-up time, lasted about 2.5 hours. The torrefaction

temperatures were generally lower than the furnace temperature setting (290

°C) except that of impregnated wood, for which the temperature fluctuated

from 300 °C up to 327 °C. This is likely due to the presence of high contents

of metal-based preservatives which could alter the thermal degradation

behavior by enabling sustained smoldering during heating. The higher

temperature in experiment with the impregnated stemwood led to a much

higher degree of thermal decomposition of feedstock and already a pyrolysis

occurred. This was reflected by the lower yield of char (47%), compared to

torrefaction from the other four feedstocks (85 and 91%).

50

Figure 18. Temperature profiles of torrefaction processes with the indicated feedstocks (Paper IV).

As shown in Figure 19. Among the five feedstocks, particle board has the

highest concentration for both PCDDs and PCDFs, while the concentrations

of PCDDs and PCDFs in the other four feedstocks are much lower. For the

torrefied products, highest PCDFs concentration (sum of gas, liquid and

chars) were found in impregnated stemwood followed by those from particle

board. For PCDDs, the highest concentrations were found after torrefaction of

particle board. Comparison of concentrations in the total torrefied products

with those in untreated materials indicated that formation of PCDFs during

torrefaction were insubstantial.

It has been shown that in heterogeneous pathways involving metal catalysts,

de novo synthesis is one of the dominant pathways of PCDF formation,

whereas PCDDs mostly originate from precursor pathway, with limited

contributions from de novo pathways (Hell et al., 2001; Wikström et al.,

2004). Hence, a PCDD/PCDF ratio <1 is often regarded as a characteristic of

de novo synthesis. Precursor-dominated formation, in contrast, often leads to

a PCDD/PCDF ratio >1. Thus, the dominance of PCDDs over PCDFs we

observed in the products indicates that precursor pathways were most

20

60

100

140

180

220

260

300

340

0 0,5 1 1,5 2 2,5

Te

mp

er

atu

re

(⁰C

)

Time (h)

Particle board

Cassava stems

ImpregnatedstemwoodBark

Stemtwood

Heating Torrefaction

51

important in our torrefaction process. Regarding relative distributions in the

torrefaction products, PCDDs and PCDFs were found predominantly in chars,

in contrast to MAP products, where chlorinated compounds were mainly

found in oil products. This difference in relative distributions in various

phases is further discussed in the following section.

Homologue profiles of torrefaction products varied from sample to sample.

For torrefaction of stemwood, bark and cassava stems, low chlorinated

compounds were dominant. For impreganated stemwood, MoCDF exclusively

dominated the homologue profile, accounting for 94-99% of the total PCDFs.

The homologue profile of torrefied particle board was dominated mainly by

the highly chlorinated PCDDs, with hexa- to octa-CDD accounting for 89-94%

of the total PCDDs. This could be related to high concentrations of hepta- and

octa-CDD in the feedstock, which might originate from PCP preservative (Li

et al., 2012).

The less chlorinated compounds have a higher tendency to volatize than

highly chlorinated compounds, which tend to be more preferentially retained

in char. This pattern can be clearly seen in results of bark, cassava stem and

particle board torrefaction experiments (Figure 20). The results regarding the

partitioning of highly and low chlorinated compounds between volatile and

char products were similar to those observed in our MAP study (Paper II), and

could be related to the differences in their vapor pressures.

PCDDs and PCDFs found in torrefied products could partly originate from

physical transfer directly from the input feedstock via distillation, e.g.,

evaporation followed by condensation and/or adsorption. In addition, PCDDs

and PCDFs in torrefied products appear to have partly originated from

formation during thermal processes, as their concentrations were higher in

the torrefaction products (sum of gas, liquid and solid) than in the feedstock.

Moreover, dechlorination and/or degradation of highly chlorinated

compounds could occur during torrefaction. For example, the relative

percentage of OCDD was higher in untreated particle board than in its

torrefied products, while relative abundances of HxCDD and HpCDD were

higher in torrefied particle board than in the feedstock.

52

Figure 19. Concentrations and distributions of PCDDs and PCDFs in the five raw feedstocks and their torrefaction products (ng kg -1). Error bars represent ± 1 SD (Paper IV)

0

100

200

300

400

500

Stemwood Bark Impregnatedstemwood

Cassavastems

Particleboard

ng

kg

-1

PCDF

gas liquid char Feedstock

2000

10000

18000

PCDD

Gas Liquid Char Feedstock

200

400

600

0

20

40

Stemwood Bark Impregnatedstemwood

Cassavastems

Particleboard

ng

kg

-1

53

Figure 20. Relative abundance of each homologue in gas, liquid and solid torrefaction products of bark, cassava stems and particle board (Paper IV).

0

0,5

1

Particle board

Gas Liquid Char

0

0,5

1

Cassava stems

0

0,5

1

Bark

54

4.4. Toxicity equivalent values

The TEQ values of the examined MAP and torrefaction products were

generally quite low, except those of particle board (Table 4). Volatiles and

chars accounted for most of the TEQ in MAP and torrefaction products,

respectively. Based on char weight, these TEQ concentrations are

substantially lower than the Swedish guideline values for dioxins in solid

materials intended for sensitive land use (50 pg TEQ g-1) (Elert et al., 1997).

Although there are no specific guidelines for TEQ concentrations in bio-oil,

they were much lower than those reported in wood combustion (7.3-22.8 ng

I-TEQ kg FUEL-1) (Lavric et al., 2004). The total TEQ of torrefaction products

of particle board (47 ng I-TEQ kg fuel-1) was the same order of magnitude as

those observed in particle board combustion products (Tame et al., 2007), and

was about 7 times higher than those of untreated feedstock (6 ng I-TEQ kg

fuel-1). For management and controlling of POPs pollution in thermal

treatment of biomass, caution should be taken on utilization of char products

especially when using contaminated biomass as feedstock.

Table 4. I-TEQ values of MAP and torrefaction products from various feedstocks (ng I-TEQ kg FUEL

-1).

MAP Torrefaction

Gas Liquid Char Gas Liquid Char

Stemwood 0.006 0.03 0.01 0.06 0.05 0.7

Bark 0.006 0.2 0.102 0.07 0.05 0.7

Impregnated stemwood

0.009 0.05 0.07 0.2 0.1 0.4

Cassava stems - - - 0.07 0.06 0.6

Particle board - - - 1 1 44

55

4.5. Effects of chemical composition of feedstocks

This section summarizes the results presented in Paper II-IV concerning

effects of feedstock properties on dioxin formation. As observed in both MAP

and torrefaction experiments, the formation of chlorinated organics varied

depending on feedstock types and their composition. Chlorine contents of the

fuel is known to be a crucial factor in the formation of PCDDs, PCDFs and

PCNs in processes such as waste incineration. Although trace amounts of

chlorine were present in the stemwood, bark and impregnated wood we used,

the relatively high amounts of PCDDs and PCDFs in bark MAP products,

compared to stemwood and impregnated wood products (Paper II) suggest

that chlorine could also play a key role in their formation during MAP

However, our studies also showed that levels of chlorine inputs and amounts

of PCDDs and PCDFs formed were not always positively correlated. For

example, formation of PCDDs and PCDFs in the torrefaction of cassava stems

were low although it had higher chlorine contents than the other four terrified

feedstocks (Paper IV). These results indicate that the feedstock’s chlorine

content may be important, but it is not the only factor that governs formation

of chlorinated compounds.

The active chlorine species that may be present must be considered when

addressing the role of Cl in formation of chlorinated compounds (Gullett et

al., 2000; Wikström et al., 2003b). The chlorine in the feedstock can be

released mainly (>90%) as HCl, which rarely acts directly as a chlorination

agent. The presence of metal catalysts is often required for converting HCl to

Cl2. Such reactions involving metal catalysts have been reported at

temperatures as low as 150°C (Gullett et al., 2000). Further, the relatively high

Fe content in biomass may promote the formation of free chlorine (Cl2), which

is a very efficient chlorinating agent and may promote formation of PCDD and

PCDF (Stieglitz, 1998; Kim et al., 2007).

Generally, bark has higher contents of phenolic extractives than stemwood,

regardless of the tree species from which the wood is taken (Vassilev et al.,

2010). Furthermore, coniferous species are rich in terpenes and other light,

cyclic organic compounds that have relatively little thermal stability, and

might give rise to formation of aromatics. As already mentioned, phenolics,

especially chlorinated phenolics, are widely considered to be crucial

intermediates in the heterogeneous formation of polychlorinated aromatics.

Pyrolysis of lignin components has been shown to generate high levels of

phenolics (Fu et al., 2014). Whether or not these phenolics are single

(chloro)phenol molecules directly produced during lignin matrix breakdown,

or associated with other aromatic compounds cannot be determined from

results of Studies I-IV. However, in either case these phenolic compounds may

56

certainly play a role in the formation of dioxins and PCNs in the considered

systems. Although stemwood contains substantial amounts of lignin (26.0%),

its chlorine content is low, which may restrict the potential for formation of

chlorophenols and other chlorinated organics (and hence PCDD/PCDF/PCN

compounds) during its thermal treatment.

The presence of metal-based preservatives can alter the thermal degradation

behavior of biomass by enabling sustained smoldering during heating

(Richards and Zheng, 1991). This may enhance the formation of

polychlorinated compounds, as observed in our experiments with

impregnated wood (Paper II and IV). PCP as preservatives often contains

high amount of highly chlorinated PCDD as impurities of its synthesis. PCDD,

mainly OCDD can be readily formed under pyrolysis of PCP (Altwicker et al.,

1990). The presence of high concentration of PCDDs in the torrefaction

products of particle board (Paper IV) could partly originate from

volatilization and/or re-condensation of volatized PCDD onto the char

surfaces.

4.6. MAP vs torrefaction: the importance of physical dynamics

This section compares our MAP and torrefaction setups from a physical

dynamics perspective, based on summarized findings of Paper II, III and

IV. To facilitate the discussion on the specifics of low-temperature

thermochemical processes occurred in MAP and torrefaction, typical

combustion condition is also included in the following comparison.

In MAP experiments, POPs were predominantly found in the pyrolysis oils,

while in the torrefaction experiments large fractions of POPs were found in

solid chars and small fractions in volatiles. We proposed that some factors

related to physical dynamics could influence the relative distribution of POPs

in gas, liquid and solid products. MAP and torrefaction both involve low

temperature regimes, but other process parameters substantially differ

between the two processes, including the heating rate, residence time and air

pressure (vacuum in MAP vs N2 flow in torrefaction), which could

substantially affect POP formation characteristics. Generally, chlorinated

organics may form either in the gas phase (vapor) homogeneously, or on solid

surfaces such as a degraded char matrix (in MAP and torrefaction) or on soot

or fly ash surfaces (in combustion) via heterogeneous reactions. In reactions

involving (chloro)phenol precursors, the formation depends strongly on

precursor concentrations and residence times, which are influenced (inter

57

alia) by the pressure in the reactor (vacuum in MAP vs N2 supply in

torrefaction). Figure 21 illustrates some of the major routes whereby PCDDs,

PCDFs and PCNs (collectively referred to as dioxins in the figure) can be

formed and transferred in the three types of thermal processes.

58

Figure 21. Schematic diagram of formation and transformation paths of dioxins in combustion, MAP and torrefaction. Lines in red represent important

routes for transformation of organic compounds.

quenching to <400 °C de novo/precursor

condensation > 600 °C O2, Δt≈100° C/s

MAP

Combustion

Volatiles + O2 - CO2, CO, Nox, H2O

- PICs

Gas phase precursor reaction

> 650 °C Dioxins (Primary formation)

chlorination /dechlorination quenching to <400 °C

Dioxins (Secondary formation)

Volatiles Evaporate quickly due to the vacuum

de novo

catalytic

- Non-condensables (CO2, CO etc.) - Condensables with low vapor pressure (precursors, H2O etc)

Dioxins in non- or condensables

Precursors(in gas)

React with carbon matrix

end up in flue gas

Precursors

Dioxins

200-300 °C N2,Δt ≈ 0.1 ° C/s

Dioxins (in chars)

up to 200° C <<atm, Δt≈1 ° C/s

Torrefaction

Degradation

Volatiles

Post combustion zone/condensors/PUF

Degradation

Evaporate quickly due to vacuum

- Non-condensables (CO2, CO etc.)

- Condensables (with high vapor pressure)

Biomass

Lower fraction evaporated than in MAP

Chars

React with carbon matrix

Chars

Dioxins on char surface

Chars

Reaction chamber

CO2, CO, Nox, H2O

59

In most combustion scenarios, temperatures range from 600 to 1000s °C.

During the initial stage as the temperature rises (at heating rates typically

above 100 K/sec), volatilization occurs, resulting in gas phase products

containing volatiles and water vapor. The volatile substances will continue

reacting with O2 to form CO2, CO, NOx etc. The products of incomplete

combustion (PICs), including precursor species formed in gas phase, will

undergo numerous reactions as the temperature and oxygen concentration

are sufficiently high. Dioxins are formed from primary processes in

combustion chambers. As volatile products pass through the post-combustion

zone with quenching conditions at temperatures below 400 °C, secondary

formation processes involving even more complex pathways, such as

condensation of precursors in gas phase will occur. The char residues and fly

ash obtained from primary combustion can provide an organic matrix for de

novo formation of dioxins. The presence of transition metal catalyst can also

promote their formation.

For our MAP experiments the temperature was increased to 200 °C at a

heating rate of about 1 °C/sec. The system used, with a connected vacuum

unit, provides much lower pressure (approximately a tenth of the atmospheric

pressure) than typical combustion conditions. This results in faster

evaporation of volatiles from the surface of the char matrix. Hence, most

primary precursors that were formed from biomass degradation readily

evaporated and ended up in the pyrolysis condensates. Only a minor fraction

remained on the char matrix. This could explain why we found lower amounts

of PCDD/Fs in chars than in oil condensates. In addition, in MAP with low

temperature and oxygen-deficient conditions, the gas phase reactions

involved in PCDD/F formation could be much less complex than in

combustion, where the solid carbonaceous residues generated are highly

reactive.

In the torrefaction experiments, the temperature reached 200-300 °C at a

heating rate of 0.1 °C/s, 10-fold slower than in the MAP setup. With N2

continuously introduced, and a vacuum unit connected at the outlet, the

volatile organics were constantly “scavenged”, hence concentrations of volatile

organics in gas phase inside the reactor were much lower than in combustion

processes with no inflow of N2. Concentrations of volatiles inside the

torrefaction reactor were much higher than those in the MAP system, and the

residence time of precursor compounds was longer. Moreover, the

combination of low torrefaction temperatures and pressures hindered

volatilization of some precursor compounds with relatively high boiling

points, in contrast to MAP where the precursors evaporated at temperatures

below their boiling points due to the vacuum. Consequently, during

60

torrefaction the precursor compounds had a higher tendency to remain in

solid carbon matrices, allowing PCDD and PCDF formation reactions. The

torrefaction temperature was also too low to make the PCDDs and PCDFs

present in the raw feedstock evaporate into gas phase. This could partly

explain the higher proportions of PCDDs and PCDFs in solid torrefied

products and lower proportions in condensates compared to corresponding

proportions in MAP products.

The water contents of the feedstocks also differed between our MAP and

torrefaction experiments. For example, in torrefaction, the materials were

pre-dried and degradation of relatively reactive saccharide components could

occur in early stages of the process, while in MAP experiments raw feedstocks

were directly introduced without pre-drying and water evaporated at the

beginning of the pyrolysis. Whether or not the presence of water vapor affects

PCDD and PCDF formation is unclear. Nevertheless, the solid residues

generated in both MAP and torrefaction contained unconverted organic

carbon which can adsorb organochlorines. Moreover, rates of adsorption and

desorption strongly depend on the atmospheric conditions. In MAP with

vacuum conditions desorption could be favored, whereas in torrefaction

adsorption may prevail. Regarding the carbon balance in volatiles and solid

products, the organic volatiles generated in combustion often contain more

than 90% of the initial carbon, while in our MAP experiments the char yields

were 35-46%. In torrefaction, normally more than 90% of carbon remains in

solid phase. The differences in yields of thermochemical products in different

processes will certainly influence the distribution of dioxins among phases.

61

5. Conclusions and future perspectives

The major conclusions from the studies underlying this thesis can be

summarized as follows:

In PLE extraction of POPs from thermally treated biomass for

simultaneous determination by GC/MS, solvents with high polarity

should not be used due to matrix interference from the high co-

extraction of thermally degraded lignocellulosic compounds. Raw

wood is less solvent-dependent and comparable results can be

obtained with polar and non-polar solvents.

In MAP experiments, POPs were predominantly retained in pyrolysis

oils, while in torrefaction experiments large fractions of POPs were

observed in solid chars with little in volatiles. These findings suggest

that physical dynamic conditions (particularly heating rate and

pressure in the reactor) could govern distributions of POPs in gas,

liquid and solid products. The abundances of POPs in non-

condensable gas products were relatively low in both MAP and

torrefaction experiments. This information could facilitate the

development of strategies for controlling gas phase emissions of

organic pollutants, and utilization of different thermally treated

products.

In both MAP and torrefaction experiments, highly chlorinated

congeners with low volatility tend to be retained in chars, whereas less

chlorinated congeners tend to volatize into the gas phase.

Isomer patterns of MAP products are more selective than patterns

observed in most combustion studies, indicating less complex

formation pathways in the low temperature and oxygen deficient

conditions used in MAP than in combustion. However, in our opinion,

the complexity of formation mechanisms involving heterogeneous

routes, as well as the uncertainties in many thermochemical and rate

parameters make it difficult to make a conclusive statement about the

contribution of each pathway. Nevertheless, the presence of isomers

with low thermodynamic stability in MAP suggests that the formation

pathways of PCDDs, PCDFs and PCNs could be governed not only by

thermodynamic factors but also kinetic factors and the reactivity of

precursor intermediates.

62

Chemical composition of feedstock, particularly its contents of

chlorine and catalytically active metals (mainly Cu and Fe), plays key

roles in PCDD, PCDF and PCN formation. Chlorine is not the only

important factor, as shown by the low concentrations of PCDD/Fs in

torrefaction products of cassava stems (which have high Cl contents).

The abundances of PCDDs, PCDFs and PCNs depend on complex

interactions between feedstock-related and process variables.

Although experiments can be designed to evaluate the effect of

individual variables, the influence on dioxin formations solely based

on knowledge of the feedstock content are difficult to apply in a

predictive manner.

Thermal conversion of biomass with high levels of chlorinated organic

contaminants such as PCP can result in high formation of dioxins

compared to clean wood. For management and recycling of

contaminated waste wood, controlled incineration still appears to be

most appropriate option for dioxin destruction. In addition, it may be

necessary to identify and investigate the particleboard still‑in‑use

and in demolition containing PCP preservatives to prevent dioxin

contamination.

Future perspectives

The presented studies indicate that (chloro)phenoxyl intermediates

could be important precursors in POP formation. Chlorination

patterns of precursors could play key roles in final PCDD/F profiles.

Detailed information on PCPhs concentrations and isomer patterns

in feedstock and thermally treated products, could be useful for fully

elucidating the role of PCPhs as precursors. Also, formation of both

PCDFs and PCNs appeared to proceed via chlorination of non- or low

chlorinated compounds. Quantification of non-chlorinated DD, DF,

CN and MoCN would be helpful to clarify the formation mechanisms

involving chlorination of non-chlorinated compounds.

In a real scenario of torrefaction at full scale, the torrefaction gas is

generally combusted and the heat is typically used for biomass drying

and heating during the torrefaction process. This differs from the

experimental setup in our bench-scaled torrefaction where the gas

products were removed directly by partial vacuum. The torrefaction

gas generally has complex composition, consisting of a permanent gas

fraction and a condensable fraction. Whether or not the flue gas used

for pre-drying biomass affects dioxin formation requires further

investigation.

63

As speculated in Chapter 7, the vacuum applied in the MAP system

could have contributed to differences in dioxin distributions among

products observed in the MAP and torrefaction experiments.

However, more experiments are required to draw solid conclusions

regarding these differences. For example, conducting MAP under N2

using the same feedstock or torrefaction under vacuum might be

helpful for comprehensive comparison of the results under different

operating conditions.

64

Acknowledgements

First and foremost, I would like to express my gratitude to my supervisor Stina

Jansson for giving me this opportunity, for sharing your great expertise in the

field of organic pollutants in thermochemical processes, for supporting me on

all scientific conferences and field studies. Thanks for always being available

on the helpful consultations. Thanks to my co-supervisor Peter Haglund and

Silvia Larsson for the great guidance and inspiration. Thanks also to my

external mentor Mariusz from Energy Centre of the Netherlands for the

scientific guidance and enthusiastic discussion. My coauthors are

acknowledged for all the contribution of publications. I would like to

acknowledge Bio4Energy (www.bio4energy.se), a strategic research

environment appointed by the Swedish government, for the financial support

of this work.

There are many colleagues that deserve special recognition for contributing to

this project. Big thanks go to Per Lilielind for help with the GC/MS analysis

and discussions. I would like to thank Mark and Vitaliy from York University,

Magnus from SLU for the help in numerous ways succeeding with my field

experiments. Thanks are due to Mats Tysklind for supporting me in GloCom

project. Mar Edo for being great companion and support throughout the

years. Thanks go to all the “combustion” girls Mirva, Lisa and Eva for the

fantastic accompany and strong support. Thanks go to Maria for instructing

me on sample cleanup and Staffan for helping me to get a quick start on

Masslynx.

All staff, fellow students and friends, I sincerely thank you for the fantastic

time, lunches, “fika” and laughs. Thanks go to Sandra for the most memorable

time and for your contagious smile, always shining in the northern darkness.

I wish to thank Cathrin, Christine (My punch-out partner, Chantons et buvons

!!), Kristin (hot as a fireball), Joao, Matyas, Jana, Mandana, Meldi, Lan,

Marcus, Sherry and all the people at MKL for all your support and the

wonderful time we have spent. My Chinese friends Chaojun, Yaozong, Junfang

and Jin are specially acknowledged for making me feel at homeland. Thank

all the people surrounding for making it such a nice place to work and live

with.

Finally, I would like to thank my family for the incredible support and

patience. Thanks my mother for the endless love. Thanks Ping for always

supporting my pursuits. I would like to express my deepest gratitude to my

fantastic son Linus. I wish all the success and peace that you deserve.

65

References

Addink, R., Altwicker, E.R., 1998. Role of copper compounds in the de novo

synthesis of polychlorinated dibenzo-p-dioxins/dibenzofurans. Environmental Engineering Science 15, 19-27.

Addink, R., Olie, K., 1995. Role of oxygen in formation of polychlorinated dibenzo-p- dioxins/dibenzofurans from carbon on fly ash. Environ. Sci. Technol. 29, 1586-1590.

Altarawneh, M., Dlugogorski, B.Z., Kennedy, E.M., Mackie, J.C., 2009. Mechanisms for formation, chlorination, dechlorination and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Prog. Energ. Combust. 35, 245-274.

Altwicker, E.R., Konduri, R.K.N.V., Milligan, M.S., 1990. The role of

precursors in formation of polychloro-dibenzo-p-dioxins and polychloro-dibenzofurans during heterogeneous combustion. Chemosphere 20, 1935-1944.

Armstrong, D.R., Cameron, C., Nonhebel, D.C., Perkins, P.G., 1983. Oxidative coupling of phenols. Part 8. A theoretical study of the coupling of phenoxyl radicals. Journal of the Chemical Society, Perkin Transactions 2, 575-579.

Bacher, R., Swerev, M., Ballschmiter, K., 1992. Profile and pattern of monochloro- through octachlorodibenzodioxins and -dibenzofurans in chimney deposits from wood burning. Environ. Sci. Technol. 26, 1649-1655.

Basu, P., 2010. Chapter 3 - Pyrolysis and Torrefaction. in: Basu, P. (Ed.).

Biomass Gasification and Pyrolysis. Academic Press, Boston, pp. 65-96.

Basu, P., 2013. Biomass gasification, pyrolysis and torrefaction- Practical design and theory, 2nd edition ed. Elsevier Inc., CA, USA.

Bergman, P.C.A., Boersma, A.R., Zwart, R.W.R., Kiel, J.H.A., 2005. Torrefaction for biomass co-firing in existing coal-fired power stations. . Report ECN-C-05-013.

Bilgic, E., Yaman, S., Haykiri-Acma, H., Kucukbayrak, S., 2016. Is torrefaction

of polysaccharides-rich biomass equivalent to carbonization of lignin-rich biomass? Bioresource Technology 200, 201-207.

Born, J.G.P., Louw, R., Mulder, P., 1993a. Fly ash mediated (oxy)chlorination of phenol and its role in PCDD/F formation. Chemosphere 26, 2087-2095.

66

Born, J.G.P., Mulder, P., Louw, R., 1993b. Fly ash mediated reactions of phenol and monochlorophenols: Oxychlorination, deep oxidation, and condensation. Environ. Sci. Technol. 27, 1849-1863.

Budarin, V.L., Clark, J.H., Lanigan, B.A., Shuttleworth, P., Breeden, S.W.,

Wilson, A.J., Macquarrie, D.J., Milkowski, K., Jones, J., Bridgeman, T., Ross, A., 2009. The preparation of high-grade bio-oils through the controlled, low temperature microwave activation of wheat straw. Bioresour. Technol. 100, 6064-6068.

Budarin, V.L., Clark, J.H., Lanigan, B.A., Shuttleworth, P., Macquarrie, D.J., 2010. Microwave assisted decomposition of cellulose: A new thermochemical route for biomass exploitation. Bioresour. Technol. 101, 3776-3779.

Demirbaş, A., Arin, G., 2002. An overview of biomass pyrolysis. Energy Sources 24, 471-482.

ECS, 2006. Stationary source emissions - Determination of the mass

concentration of PCDDs/PCDFs (EN1948: 1-3). European Committee for Standardization.

Elert, M., Jones, C., Norman, F., 1997. Model and data used for generic guideline values for contaminated soils in Sweden. Swedish Environmental Protection Agency

ER, 2015. Energy in Sweden 2015. Swedish Energy Agency, Arkitektkopia, Bromma.

Evans, C.S., Dellinger, B., 2003. Mechanisms of dioxin formation from the

high-temperature pyrolysis of 2-chlorophenol. Environ. Sci. Technol. 37, 1325-1330.

Falandysz, J., 1998. Polychlorinated naphthalenes: An environmental update. Environmental Pollution 101, 77-90.

Fan, J., De bruyn, M., Budarin, V.L., Gronnow, M.J., Shuttleworth, P.S., Breeden, S., Macquarrie, D.J., Clark, J.H., 2013. Direct microwave-assisted hydrothermal depolymerization of cellulose. J Am Chem Soc 135, 11728-11731.

Fu, D., Farag, S., Chaouki, J., Jessop, P.G., 2014. Extraction of phenols from

lignin microwave-pyrolysis oil using a switchable hydrophilicity solvent. Bioresour. Technol. 154, 101-108.

67

Ghorishi, S.B., Altwicker, E.R., 1996. Rapid formation of polychlorinated dioxins/furans during the heterogeneous combustion of 1,2-dichlorobenzene and 2,4-dichlorophenol. Chemosphere 32, 133-144.

Gullett, B.K., Bruce, K.R., Beach, L.O., 1990. The effect of metal catalysts on

the formation of polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran precursors. Chemosphere 20, 1945-1952.

Gullett, B.K., Sarofim, A.F., Smith, K.A., Procaccini, C., 2000. The role of chlorine in dioxin formation. Process Saf. Environ. 78, 47-52.

Hell, K., Stieglitz, L., Dinjus, E., 2001. Mechanistic aspects of the de-novo synthesis of PCDD/PCDF on model mixtures and MSWI fly ashes using amorphous 12C- and 13C-labeled carbon. Environ. Sci. Technol. 35, 3892-3898.

Hisham, M.W.M., Benson, S.W., 1995. Thermochemistry of the deacon

process. Journal of Physical Chemistry 99, 6194-6198.

Hoffmann, D., Weih, M., 2005. Limitations and improvement of the potential utilisation of woody biomass for energy derived from short rotation woody crops in Sweden and Germany. Biomass Bioenerg. 28, 267-279.

Hooth, M.J., Nyska, A., Fomby, L.M., Vasconcelos, D.Y., Vallant, M., DeVito, M.J., Walker, N.J., 2012. Repeated dose toxicity and relative potency of 1,2,3,4,6,7-hexachloronaphthalene (PCN 66) 1,2,3,5,6,7-hexachloronaphthalene (PCN 67) compared to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for induction of CYP1A1, CYP1A2 and thymic atrophy in female Harlan Sprague-Dawley rats. Toxicology 301, 85-93.

Jansson, S., Fick, J., Marklund, S., 2008. Formation and chlorination of

polychlorinated naphthalenes (PCNs) in the post-combustion zone during MSW combustion. Chemosphere 72, 1138-1144.

Kim, D.H., Mulholland, J.A., Ryu, J.Y., 2007. Chlorinated naphthalene formation from the oxidation of dichlorophenols. Chemosphere 67, S135-S143.

Krook, J., Mårtensson, A., Eklund, M., 2004. Metal contamination in recovered waste wood used as energy source in Sweden. Resources, Conservation and Recycling 41, 1-14.

Krook, J., Mårtensson, A., Eklund, M., Libiseller, C., 2008. Swedish recovered

wood waste: Linking regulation and contamination. Waste Management 28, 638-648.

68

Lam, W.C., Kwan, T.H., Budarin, V.L., Mubofu, E.B., Fan, J., Lin, C.S.K., 2015. Pretreatment and Thermochemical and Biological Processing of Biomass. Introduction to Chemicals from Biomass: Second Edition, pp. 53-88.

Lavric, E.D., Konnov, A.A., De Ruyck, J., 2004. Dioxin levels in wood

combustion - A review. Biomass Bioenerg. 26, 115-145.

Li, C., Zheng, M., Zhang, B., Gao, L., Liu, L., Zhou, X., Ma, X., Xiao, K., 2012. Long-term persistence of polychlorinated dibenzo-p-dioxins and dibenzofurans in air, soil and sediment around an abandoned pentachlorophenol factory in China. Environmental Pollution 162, 138-143.

Libit, L., Hoffmann, R., 1974. Toward a detailed orbital theory of substituent effects: Charge transfer, polarization, and the methyl group. Journal of the American Chemical Society 96, 1370-1383.

Liljelind, P., Söderström, G., Hedman, B., Karlsson, S., Lundin, L., Marklund,

S., 2003. Method for multiresidue determination of halogenated aromatics and PAHs in combustion-related samples. Environ. Sci. Technol. 37, 3680-3686.

Liu, K., Pan, W.P., Riley, J.T., 2000. Study of chlorine behavior in a simulated fluidized bed combustion system. Fuel 79, 1115-1124.

Manuel, G.-P., Metcalf, J., 2008. The formation of polyaromatic hydrocarbons and dioxins during pyrolysis: A review of the literature with descriptions of biomass composition, fast pyrolysis technologies and thermochemical reactions. Washington State University, Pullman.

McKendry, P., 2002. Energy production from biomass (part 1): Overview of

biomass. Bioresource Technology 83, 37-46.

Mohan, D., Pittman Jr, C.U., Steele, P.H., 2006. Pyrolysis of wood/biomass for bio-oil: A critical review. Energ. Fuel. 20, 848-889.

Mushtaq, F., Mat, R., Ani, F.N., 2014. A review on microwave assisted pyrolysis of coal and biomass for fuel production. Renew. Sust. Energ. Rev. 39, 555-574.

Nordin, A., Pommer, L., Nordwaeger, M., Olofsson, I., 2013. Biomass

conversion through torrefaction. Technologies for Converting Biomass to Useful Energy. CRC Press, pp. 217-244.

69

Oh, J.E., Gullett, B., Ryan, S., Touati, A., 2007. Mechanistic relationships among PCDDs/Fs, PCNs, PAHs, CIPhs, and CIBzs in municipal waste incineration. Environ. Sci. Technol. 41, 4705-4710.

Pekárek, V., Grabic, R., Marklund, S., Punčochář, M., Ullrich, J., 2001. Effects

of oxygen on formation of PCB and PCDD/F on extracted fly ash in the presence of carbon and cupric salt. Chemosphere 43, 777-782.

Piao, C., Monlezun, C.J., Wang, J.J., Groom, L.H., 2011. Recycling of pentachlorophenol-treated southern pine utility poles. Part I: Preservative retention and mechanical properties. Forest Products Journal 61, 38-45.

Procaccini, C., Bozzelli, J.W., Longwell, J.P., Sarofim, A.F., Smith, K.A., 2003. Formation of chlorinated aromatics by reactions of Cl. Cl2, and HCl with benzene in the cool-down zone of a combustor. Environ. Sci. Technol. 37, 1684-1689.

Richards, G.N., Zheng, G., 1991. Influence of metal ions and of salts on

products from pyrolysis of wood: Applications to thermochemical processing of newsprint and biomass. J. Anal. Appl. Pyrol. 21, 133-146.

Robinson, J.P., Kingman, S.W., Baranco, R., Snape, C.E., Al-Sayegh, H., 2010. Microwave pyrolysis of wood pellets. Ind. Eng. Chem. Res. 49, 459-463.

Ryu, J.Y., Choi, K.C., Mulholland, J.A., 2006. Polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofurna (PCDF) isomer patterns from municipal waste combustion: Formation mechanism fingerprints. Chemosphere 65, 1526-1536.

Ryu, J.Y., Kim, D.H., Jang, S.H., 2013. Is chlorination one of the major

pathways in the formation of polychlorinated naphthalenes (PCNs) in municipal solid waste combustion? Environ. Sci. Technol. 47, 2394-2400.

Ryu, J.Y., Mulholland, J.A., Dunn, J.E., Iino, F., Gullett, B.K., 2004. Potential role of chlorination pathways in PCDD/F formation in a municipal waste incinerator. Environ. Sci. Technol. 38, 5112-5119.

Ryu, J.Y., Mulholland, J.A., Kim, D.H., Takeuchi, M., 2005. Homologue and isomer patterns of polychlorinated dibenzo-p-dioxins and dibenzofurans from phenol precursors: Comparison with municipal waste incinerator data. Environ. Sci. Technol. 39, 4398-4406.

Sandberg, D., Haller, P., Navi, P., 2013. Thermo-hydro and thermo-hydro-

mechanical wood processing: An opportunity for future environmentally friendly wood products. Wood Material Science and Engineering 8, 64-88.

70

Schneider, M., Stieglitz, L., Will, R., Zwick, G., 1998. Formation of polychlorinated naphthalenes on fly ash. Chemosphere 37, 2055-2070.

Shuttleworth, P., Budarin, V., Gronnow, M., Clark, J.H., Luque, R., 2012. Low

temperature microwave-assisted vs conventional pyrolysis of various biomass feedstocks. Journal of Natural Gas Chemistry 21, 270-274.

Sidhu, S.S., Maqsud, L., Dellinger, B., Mascolo, G., 1995. The homogeneous, gas-phase formation of chlorinated and brominated dibenzo-p-dioxin from 2,4,6-trichloro- and 2,4,6-tribromophenols. Combustion and Flame 100, 11-20.

Song, G.J., Kim, S.H., Seo, Y.C., Kim, S.C., 2008. Dechlorination and destruction of PCDDs/PCDFs in fly ashes from municipal solid waste incinerators by low temperature thermal treatment. Chemosphere 71, 248-257.

Stieglitz, L., 1998. Selected topics on the de novo synthesis of PCDD/PCDF on

fly ash. Environ. Eng. Sci. 15, 5-18.

Sundqvist, J.-O., 2009. Treated wood in circulation : recommendations for waste management (In Swedish). IVL Swedish Environmental Research Institute, Stockholm.

Tame, N.W., Dlugogorski, B.Z., Kennedy, E.M., 2007. Formation of dioxins and furans during combustion of treated wood. Prog. Energ. Combust. 33, 384-408.

Tanger, P., Field, J.L., Jahn, C.E., DeFoort, M.W., Leach, J.E., 2013. Biomass

for thermochemical conversion: targets and challenges. Frontiers in Plant Science 4.

UNEP, 2001. Stockholm convention on Persistent Organic Pollutants (POPs). United Nations Environment Programme.

Uslu, A., Faaij, A.P.C., Bergman, P.C.A., 2008. Pre-treatment technologies, and their effect on international bioenergy supply chain logistics. Techno-economic evaluation of torrefaction, fast pyrolysis and pelletisation. Energy 33, 1206-1223.

Van den Berg, M., Birnbaum, L.S., Denison, M., De Vito, M., Farland, W.,

Feeley, M., Fiedler, H., Hakansson, H., Hanberg, A., Haws, L., Rose, M., Safe, S., Schrenk, D., Tohyama, C., Tritscher, A., Tuomisto, J., Tysklind, M., Walker, N., Peterson, R.E., 2006. The 2005 World Health Organization reevaluation

71

of human and mammalian toxic equivalency factors for dioxins and dioxin-like compounds. Toxicol. Sci. 93, 223-241.

Van der Stelt, M.J.C., Gerhauser, H., Kiel, J.H.A., Ptasinski, K.J., 2011.

Biomass upgrading by torrefaction for the production of biofuels: A review. Biomass Bioenerg. 35, 3748-3762.

Wang, Z.Y., Zhai, Z.C., Wang, L.S., Chen, J.L., Kikuchi, O., Watanabe, T., 2004. Prediction of gas phase thermodynamic function of polychlorinated dibenzo-p-dioxins using DFT. Journal of Molecular Structure: THEOCHEM 672, 97-104.

Vassilev, S.V., Baxter, D., Andersen, L.K., Vassileva, C.G., 2010. An overview of the chemical composition of biomass. Fuel 89, 913-933.

Weber, R., Hagenmaier, H., 1999. Mechanism of the formation of

polychlorinated dibenzo-p-dioxins and dibenzofurans from chlorophenols in gas phase reactions. Chemosphere 38, 529-549.

Weber, R., Iino, F., Imagawa, T., Takeuchi, M., Sakurai, T., Sadakata, M., 2001. Formation of PCDF, PCDD, PCB, and PCN in de novo synthesis from PAH: Mechanistic aspects and correlation to fluidized bed incinerators. Chemosphere 44, 1429-1438.

Weber, R., Nagai, K., Nishino, J., Shiraishi, H., Ishida, M., Takasuga, T., Konndo, K., Hiraoka, M., 2002. Effects of selected metal oxides on the dechlorination and destruction of PCDD and PCDF. Chemosphere 46, 1247-1253.

Weber, R., Sakurai, T., 2001. Formation characteristics of PCDD and PCDF

during pyrolysis processes. Chemosphere 45, 1111-1117.

Wiater, I., Born, Jan G.P., Louw, R., 2000. Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K. European Journal of Organic Chemistry 2000, 921-928.

Wikström, E., Marklund, S., 2000. Secondary formation of chlorinated dibenzo-p-dioxins, dibenzofurans, biphenyls, benzenes, and phenols during MSW combustion. Environ. Sci. Technol. 34, 604-609.

Wikström, E., Ryan, S., Touati, A., Gullett, B.K., 2003a. Key parameters for de

novo formation of polychlorinated dibenzo-p-dioxins and dibenzofurans. Environ. Sci. Technol. 37, 1962-1970.

72

Wikström, E., Ryan, S., Touati, A., Gullett, B.K., 2004. In Situ Formed Soot Deposit as a Carbon Source for Polychlorinated Dibenzo-p-dioxins and Dibenzofurans. Environ. Sci. Technol. 38, 2097-2101.

Wikström, E., Ryan, S., Touati, A., Telfer, M., Tabor, D., Gullett, B.K., 2003b.

Importance of chlorine speciation on de novo formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. Environ. Sci. Technol. 37, 1108-1113.

Wolfesberger, U., Aigner, I., Hofbauer, H., 2009. Tar Content and Composition in Producer Gas of Fluidized Bed Gasification of Wood-Influence of Temperature and Pressure. Environ. Prog. Sustain. 28, 372-379.

Yan, W., Acharjee, T.C., Coronella, C.J., Vásquez, V.R., 2009. Thermal pretreatment of lignocellulosic biomass. Environ. Prog. Sustain. 28, 435-440.

Yang, Y., Mulholland, J.A., Akki, U., 1998. Formation of furans by gas-phase

reactions of chlorophenols. Symposium (International) on Combustion 27, 1761-1768.

Yin, C., 2012. Microwave-assisted pyrolysis of biomass for liquid biofuels production. Bioresour. Technol. 120, 273-284.

Zhai, Z., Wang, Z., 2005. Computational study on the relative stability and formation distribution of 76 polychlorinated naphthalene by density functional theory. Journal of Molecular Structure: THEOCHEM 724, 221-227.

Zwart, R.W.R., Van Der Drift, A., Bos, A., Visser, H.J.M., Cieplik, M.K.,

Könemann, H.W.J., 2009. Oil-Based gas washing- flexible tar removal for high-efficient production of clean heat and power as well as sustainable fuels and Chemicals. Environ. Prog. Sustain. 28, 324-335.